-snort- I remember this, Duna. xD
First off, I'm generally all-for the occasional "instrusion," as it can be interesting when the interaction is done courteously in a user-to-user way. This doesn't mean your character has to be the nicest thing in the world, but you have to keep the OOC aspect in mind to prevent any hurt feelings between users. This is where jumping into the RP actually does turn into an intrusion.
In the instance of this "attack" Clan, these users came up and stood off to the side of the group, making sideways comments about "Oh, what a wimpy Clan, they don't even notice us!"
Of course, I was playing on a kit in this instance, and we were in the middle of an announcement from the clan leader (Heavystar). Had Heavy wanted to bring the clan to challenge the intruders, I would've been fairly interested in the situation. After all, when played right, who doesn't like a good spar with another group? It can add some real excitement to your story.
However, because we'd only just started RPing, the group leader didn't want to bring us into the battle.
The attacking Clan argued that because we were RPing in Local chat, we had to acknowledge their presence and respond to their attacks.
Which, of course, isn't true at all. You can always choose whether or not to RP with someone, and so we decided to just keep our RP within our own clan, at least for the time being.
They continued arguing with us, but really in situations like that, that's when I approached those users and talked to them (in OOC, with parenthesis like this to show I wasn't RPing). I basically explained to them that our RP leader just wanted to keep our RP within the group for the time being, and that we weren't going to reply to their posts while in character. However, I added that they were welcome to stay, since we all share the maps fairly.
^ That's the way those situations should be handled, in my opinion.
And it seemed to work, because the group suddenly went from semi-hostile to "OMG a kit! Lol, ok I guess that's ok if you don't want to battle us" shortly afterward.
They did stick around for a little while longer, but soon enough they went on their way, and everybody was happy.
That's usually how I handle those situations, whenever I am able to settle down and Rp a bit. The key is keeping a cool head, and remembering that using respectful reasoning and being fair with users that try to "intrude" can diffuse many arguments to something much more pleasant.