How do you stop a teenager eating bucketloads of sweets?

Nothing wrong with being honest and explaining that sugar and fizz will cause tooth decay and health issues. It’s a fact of life!

My 24 ds has lost his sweet tooth entirely.
People’s taste can change.

We also rarely keep sweet fizzy drinks in the house (we have caffeine free Coke Zero, he has trouble sleeping and his sister is 9 so shouldn't have caffeine either), if he buys fizzy pop instead of sweets we keep it in the outside shed and bring it out at mealtimes (the dentist said sweet drinks were OK with meals!).

We do have a lot of "sacrificial foods" whether it's something we want to eat, we want to keep for meals, or we don't want him bingeing on - so we keep the sugar free fizzy in the fridge, the very large bottle in an easy to access spot and the cans for us at the back, and a small number of biscuits in the tin - if it's easy to access he can have it. DH has diabetes and needs sweet snacks on hand so he hides them in random drawers!

I have no idea - we have a 12 year old who has ADHD and sweet/appealing food is a big dopamine hit. We have more control over his money but he's a lot less biddable than other children his age.
We do restrict heavily how much money he has (he is allowed to buy sweets at the weekend but only with the cash we give him for the purpose, as the rest of his pocket money goes into his building society account). He gets school dinners (they are just served up, no buying them, which is unusual for secondary I know - it's a specialist school - they do get some choice) and he doesn't take himself to/from school.
We don't keep them in the house but he also has cravings for savoury stuff so we find all the chorizo for tomorrow's tea has been eaten!
We have expectations to get screen time (homework done, no hurting anyone, eat what we give you permission to eat) so it's framed as a positive expectation rather than "if you eat that you lose screen time" but it's a BIG battle and we have to have a lot more control than other parents might do.
We usually say yes to "can I have a biscuit" unless it's five minutes to tea, and no to "can I have a nutella sandwich for lunch" but we have to restrict how much nutella we buy and keep it hidden or he eats a whole jar with a spoon.

As far as messages we go with "if you eat now you won't be hungry for tea" and "sugar is bad for your teeth" and "we need that for tea" or occasionally "it was pasta for tea, you still have some, breakfast is tomorrow" if he's just ignored his tea and is coming back for toast and jam.

The slight restriction on snacks and "are you done, OK, breakfast is tomorrow" is on the advice of a dietician (online, but she's properly qualified), (Thalia, Picky Eater/Family Dietician).

Sorry for the essay! It's an ongoing battle and I'm sure it would be 1000x worse if he had money!

Mine grew up with the rule that one 'treat' a day was ok and they got to choose what it was (within reason). I hadn't given it much thought and understand I'd now be roasted for allowing my kids a sweet everyday but it seemed to work and even now as adults they generally follow the rule. Which makes laugh as the oldest ones are in their thirties.

I wasn't super strict when we went out for a meal or at birthday parties but still relatively strict.

I really emphasised the teeth health side of things and how bad grazing is for teeth. I generally didn't give my kids snacks.

I simply didn't keep treats in the house so there were no biscuits, crisps, fizzy drinks, sugary cereals etc to be had. I still don't but that's for my benefit.