Well, i think it is kind of annoying when some one just comes and interrupts an rp, but i don't think its right to just scream at them, whisper them and ASK them to leave first.
I personally HATE it when someone starts trying to get involved with out asking first. I remember once i was in a wolf rp, i was just rping and then some one came along and started annoying us. They were ok at first but... eventually they asked to join. They were a bright blue and red but as it was realistic, they didn't meet the requirements to join the pack. I told them that if they wanted to join, they would need to edit there char... they refused (i was the beta). As we were rping in local they tryed to join in but i told them they needed to edit first. Soon i asked them to leave and they just sat there. they were constantly trying to join in and i found this VERY frustrating. i didn't shout, i asked them over whisper if they could leave. But no. They sat there. Everyone else was beginning to get annoyed. Soon a fight broke out and we had to move because of it. I mean, the guy was probably 8, they were REALLY stupid. they didn't even know what the name of a group of wolves was, they spelt almost every word incorrectly and were swearing there heads of at the end of it. We also lost ALOT of members that day (it was a long term rp) including one of our well liked and well know original members who was always keeping the pack under control (The pack fell apart after she left).
I think that you are being rude by interrupting an rp but there was no reason for them to act like that. Yes, if they didn't want you there, they should of asked you to leave but... i wouldn't recommend trying to join an rp all ready under way, maybe try going around it in a different way. It is EXTREMELY frustrating when some idiot comes and kills your rp, But still, there was no reason for the screaming and godmodding.