There are some really unstable, bizarre people that have this ideas ponies are the devil and need nothing but a harsh, heavy hand CONSTANTLY it's surprising and unnerving.
I used to have a little Sec A who we got as a non-ridden companion as he was said to be too unsafe and dangerous for use as a child's ridden pony.
The lady we got him from thought the world of him but admitted she probably made a mistake in buying him because she felt sorry for him most of all. He'd been whacked, walloped and knocked about something shocking by idiots that had him before her.
The first time she saw him was during a family fun day in a large equestrian centre where a young girl was trotting around in the arena and the pony gave out of a few little kicks along the way. She was only very young and it knocked her confidence, she pulled up and burst into tears. Some bloke who was well known in the centre went over and in an attempt to try and spare her any more tears or embarrassment told her to step aside whilst he taught that naughty pony a lesson and promptly got on him, rode and rode and rode him around the arena to the point of near collapse. He bucked a few times - no doubt because of the weight of a grown ass man over 6ft tall on his back - and each time he did, the guy cracked him and kept forcing him to carry on. She said it was comical the first 20 seconds or so but quickly grew really uncomfortable, awkward and the mood changed totally. A woman clambered over into the arena and made him stop. She was heard through the mic on his headset saying “OK you've made your point and had your fun – get off that pony before you kill him” He was probably embarrassed or trying to make himself look even bigger so started "Oh here we go another one feeling sorry.." and she shut him down and went "GET OFF - NOW"
He got off (had to really from the sounds of it that woman meant business) but as the poor lad was being lead from the arena, he was wobbling and his legs all over the place like cooked spaghetti.
The situation in your case is flat out abusive and cruelty for sure. If you can't or aren't comfortable with saying anything outright and upfront – even if you are really – it wouldn't hurt to install a basic CCTV and catch these nasty bastards in the act and have footage to use for reference but don't share or upload to social media or anywhere.
I'm not a fan of people generally filming their issues and sharing it that way but it will definitely help if you have any footage of this kind of crap so you can let RSPCA or whoever see it and jump to respond quickly.
I don't know why it is that more “experienced” and often qualified instructors and professionals tend to be the worst and have no idea how else to do anything other than to use brute force and abuse. If that's what you need to do every day to address a so called problem pony, it's obviously not working is it?
My old little Sec A had his happy ending though fortunately. I had a feeling he may not have been entirely broken spirited as first thought and wondered if his issue with being ridden was more out of association than anything else (former owner said he would throw riders clean off over his head)
Had my daughter help with just basic groundwork, confidence building and then got her to gradually ease up until sat on him just to see whether his issue with having someone on his back was due to pain or physical discomfort or if it was more a mental association and assumption that having someone sat on him inevitably came with a bad experience.
When she swung her leg over and sat upright he pinned his ears and shifted on his feet like a grumpy old man getting ready to kick the neighbour's cat and I immediately thought “He can be ridden he just needs to reset that switch in his brain”
Sadly my daughter was too big for him to ride regularly but the farmer's daughter is a great lass, experienced rider and actually wanted to help restart him which she did and within a few weeks he was being tacked up and ridden daily without any issue whatsoever.
I was told his habit of bucking and throwing riders was nearly always when asked to move from trot to canter but being so little I couldn't ride him myself to know for sure but farmer's girl was happy to ride him in the arena. We'd already talked and I had briefed her beforehand although I was 99% certain any bucking he might do was not gonna be full blown Buckaroo whizzing her clean over his head so when she first tacked up and rode him in the arena I remember going “OK when you start picking up to a canter he might buck or throw a few kicks but DON'T REACT”
Was pretty sure he was expecting either to get a crack or have her pull him up and jump off so the aim for him to having nothing at all like he was used to.
He threw out a few very small bucks that were about as high and powerful as him lifting his arse end to fart and when it didn't get any real reaction, he tried another and on the third go she just firmly said “Hey – pack it in and stop being daft” and kept him going after which he just pulled his face a bit but that's all. If anything he looked cheesed off that his trick didn't work at all like he hoped 
Ended up signing him over to her permanently because I couldn't justify keeping him and just letting him go to waste turned out when he'd been given a second chance and new friend and rider he and I both trusted.
Only learned later he'd gone through no fewer than three different owners all experienced and involved in the world of pony club activities and yet it took a 10yr old girl to help him find a new happiness and enjoy being ridden.