I think that this is mostly a matter of self policing and trying to be a good RPer. And helping each other learn in a positive way.
If you pose or do something (like sitting down) that puts distance between you and another character in the room (due to their posing or @tp or seating), try and respect it when you RP. Pose calling out loudly to them when you speak to them. Stand up before you pick their pockets. Pose crossing the room before you whisper to them.
I know for a fact that this can be troublesome. I've forgotten that I was sitting before. I've poses standing up but forgot the stand command. Mistakes happen. But I try and to consider the room description, ambient population and my character's current situation in relation to others when reacting to them or acting on them. I think we should all try this.
I also think that you should try and pair poses with changing your @tp - especially if the @tp change reflects a change of location, positioning or activity. Just an @tp alone will inform people if they look again or enter the room but those already present have no idea of the change unless you give them one.
Also do your best to be generous. I mean, if someone walks in, sits down, then stands up and leaves and after all this is done I notice I am missing flash I will try and be generous and assume that either they were not at all responsible (maybe there was another character that got me?) or that they got me as they entered or left. I am not going to assume that they are power gaming bakas.
Hell. Even if I am sitting in a booth across the room and they are at the bar and they are sitting at the bar and I get the "You notice XXX going through your pockets) message that clearly identified them, I will assume it was a mistake on their part. Mention it oocly maybe "Hey... You forgot to stand before dipping me!" or mention it on xhelp so they can give them a reminder. But only if I KNOW. I'll also probably react as if they came over and dipped me and I caught them. and keep the RP flowing.
I'm just saying that there are too many possibilities between poses and seating and location differences to easily answer with a quick coded solution and in most cases things are legit or simple mistakes. But we can try and help each other become better players.