I too had this 3 times already some days ago. (On the same day.)
I was hosting a match server with some of my levels I made, after some people came in and dueled like crazy, a whole bunch of green sonics came in, and at first I was thinking it was some players, but they weren't even moving, and I was thinking... "You don't play then leave" So I kicked them all and they came back, same time joining at the same time they came in before, so I shut down the server due to that.
Now I host a co-op server with mysticrealm, at ZONE 2-2 during the 1st quest, those green sonics came back again, and the server was lagging so much, I had to shut it down again. (And my player limit was set to 8 as always, unless its a priv. netgame.)