http://www.project1999.org/forums/showthread.php?t=2653
For outdoor static spawns such as spectres, the rules state that if someone is sitting on the spawn point itself, they are entitled to that one mob.
In practice, it usually comes down to how quickly and how well someone is clearing an area. If mobs are staying up a significant amount of time, they are 100% within their rights to take a few after maybe trying to see what's wrong with the camper.
Is it cool to do that? Not really, to the person being supplanted. But is it cool to take a "camp" you can't fully handle, and claim the whole thing? Not really, to the person waiting.