This is quite frustrating to say the least. TopTrump has some good suggestions for cutting down on unwanted freeroll guests, especially the posting the password a couple hours prior to the start of the tournament tip.
Sunpoker.com can also set up tournaments for only specific players. The player username and account number would need to be provided.
Also, a tournament can be set up so when that only players that accumulate a certain number of poker points can enter. This has also helped and encourages players to play.
Best Regards,
Mark