A fence you never built can’t be defended. Sketch the property line before somebody wanders onto it.

A limit that has never been stated cannot be respected, because nobody knows it exists. People generally are not testing you. They are moving through their own lives and will keep going until something marks the edge, and the marking is your job rather than theirs. Resentment builds in the gap between an unspoken rule and a normal person’s behaviour.

Work out where your line actually is on one thing that keeps bothering you. Say it plainly, in advance, when nothing is wrong. Stated calmly and early, it rarely needs defending at all.