I read somewhere (I think on this wiki or on MeatBall?) about some technical society or union or website or something which had done this.
Have an invitation-only trust network, like AdvoGato. However, whoever invites you remains your "sponsor". If you should misbehave and your access is cut off, your sponsor's access is cut off also.
There's many variants to this idea. For example, the sponsorship tie could be cut after you've been a member for awhile with no problem. Or, perhaps your sponsor would be instantly invited back in by someone else, making the sponsor's punishment more of an annoyance than anything else.
Another place for variation is how the decision to cut someone off is made. Perhaps anyone "above" you in the hierarchy could cut you off at any time; i.e. a hierarchy based on how many hops separate you from the root node (the community founder/GodKing). Perhaps it requires a quorum of 3 people who are all above you in the hierarchy (closer to the GodKing) to cut someone off. Perhaps valued community members acquire multiple "sponsorships", making them immune to any single person cutting them off.
And of course, if this trust net is used to control website access, perhaps most of the site could be open, with only some parts requiring sponsorship.
Of course, a simpler system would be just to not make any change until a second user approves it. I keep forgetting that WikiWord for that feature idea..
The problem with the variant you described (sponser cut off, but can be rejoined easily) is that black hats just make sponsor chains.
Find some "trustworthy" sap who needs to send email and stuff, but happens to have some semi-trustworthy friends. Then that person links in the semi-trustworthy friends. Then pay the semi-trustworthy to sponser the untrustworthy. The semi-trustworthy keeps getting knocked on and off, but that's okay. That person can bring in 500 people easy.
If you get a few chains of the semi-trustworthy, and have redundant trusts, then there's no problem at all, for the black hats.
I do think that some web of trust network idea may be the way to go. But it's tricky figuring out how to do this right. I do not know if it can work in a way that scales to, say, a few billion people.