Being deliberately vague.
We are having trouble with a new friend of DC2 (4). New friend (NF) is 5, lives nearby and is able to access our garden freely. DC2 and NF generally play well and DC2 looks forward to playing with NF. However since they started having informal playdates we have had issues:
NF has hit and pushed DC2 a couple of times. DC2 only told me when they fell on the floor after being pushed and hurt themselves. Then DC1 told me NF had punched DC1 the day before.
NF has thrown sand at our neighbours when they were having a barbecue
NF has hidden important things in our house. We had to get NF's mum and NF walked triumphantly into our house saying "do you want me to tell you where I have put xyz?" 
NF has emptied the sandpit into the grass so we are going to have to buy more sand. Our sandpit is fixed so this has taken a sustained, concerted effort.
NF has broken some of DC1's toys - squishies pulled apart so no accident
NF has drawn on walls - not a little bit, an area of 3ft by 3ft on two different walls. It's not coming off so we will have to paint both walls.
NF doesn't listen when they are told to do or not do something - keep your voice down (shrieking upsetting DC1 who struggles with noise), get off dangerous broken fence etc.
Now we have had a lot of playdates with both my DC and I have never seen anyone who is so destructive on purpose. We have had boisterous children, clumsy children, careless children... but this is different, there is a defiance here. I am not being precious - my children are no angels and I have written on here before about my DC1 serious behaviour issues.
My question is, how would you handle this? NF can access our back garden at will and with the warm weather I keep the back door open so it's very difficult to keep NF out of the house. Sometimes I have told NF it's time to go home - NF runs away from me and I have to hold NF's hand and march them to their house very firmly. If I tell NF that they are not allowed to play in the house or upstairs I will have a fight in my hands. Basically I cannot stop NF from coming into my garden or house without putting up a fence or creating some awkwardness.
Worst thing is, NF's parents are absolutely lovely. They heard about the hitting and pushing and were horrified and disciplined NF. If I tell them about everything else they will be mortified. We live in a small community so I don't want to create any awkwardness.
Any advice? I am hoping someone has been there before and handled it beautifully
TIA
PS if you are NF's mum reading this I am sorry! You are lovely but I need help dealing with this!