Children in my care had trouble keeping bounderies until I explained to them how they do not like when others, like friends and playmates, touch or play with their things, 'without asking'.
I go on to explain and remind the kids how upset they become when their rooms get messy because others have 'not put stuff back'.
Another problem is when toys are broken when 'someone' was touching or playing with them, without permission.
So now the kids understand why they need to be careful and respectful with other people's property.
I hope this is useful to you, and saves time and energy with your current situation!