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!
I have a hard time saying NO to my children. I get to the point where I feel walked on then I blow up. Like "Katie Kaboom". My older boys refuse to clean up their own messes. I am getting to the point where I feel like throwing all of their belongings in the dumpster. Honestly If someone says that life is just peachy and that their kids obey them and listen all the time then I have a strong feeling they are about to Grow a LOOOOOOOOOOONG point nose