Human beings are inherently going to take the path of least resistance. This is why getting some things to change is like pulling teeth, especially when it comes to behavior. Habits are also very hard to break. So when you combine all of these things, getting someone to quit some negative behavior can seem almost impossible.
The difficulty increases exponentially when you allow toxic behavior. What you allow will continue. Period. Don't expect other people to do the right thing when they've been getting away with something the whole time. This applies to everyone - children, employees, students, even pets. Remember that anyone trying to take advantage of you (or a system) will continue to do so as long as there is no resistance.
How can someone cross a boundary when you don't set one? Even if you set a boundary, if you don't enforce it, some one will violate it. If they violate it and nothing happens, they will continue to do so. You cannot allow people to break your boundaries, no matter who they are. And this is a very difficult thing, especially if you're not accustomed to being assertive or if a particular person has been consistently taking advantage of you. You owe it to yourself to learn and practice assertiveness. There are plenty of resources out there. Just do it.
Set boundaries and enforce them. Otherwise, what you allow will continue.