I've never had to deal with some one crossing the line with my partner thankfully. Though I think that it would depend on exactly what they did to cross the line.
Clearly if they knew that the person was in a committed relationship there is a big issue that has to be dealt with. One being respect. I can't imagine trying to "move in" on some one who's committed to begin with but to do that to some one who I knew is beyond the pale. There are some things that can't ever be forgiven and I like to think most people know that.
For me if some one did cross a line I would want to know what the hell they were thinking when situation X took place. Were they drunk or on drugs?? The other thing I would want to know is what my partner did in return. Were they honest with me about what happened? Did they participate? That however, is a whole different ballgame I suppose.
To answer your question on what I would do if some one did cross a line.. That would be the end of whatever relationships (business or personal) I had with them and I would not want my sig other to be around them again. One trust is broken in that way I'm just not sure it can be regained. I think it just opens up to many questions about trust.
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