[Suicide Junkie]

I think closing off warps is a fair strategy right now because once they make it so you can't get in, they can't get out. That's severly limiting, especially since they can't get out to defend their unprotected sectors and you start them all rioting. They will eventually have to come out of their shell.

If they lock themselves into a small number of systems, they can stay there indefinitely.

The only thing that could possibly break the turtling is random events.
Even supernovae can be prevented by a Ring or Sphereworld, and if there are grav shield facilities on multiple planets in each system, earthquakes and planet core instabilities won't crack the shell either.
After thousands of turns, you might find that the turtled player's planets are all randomly destroyed (he can't rebuild since the ringworld uses up the star). I don't think ringworlds can be broken up by random events, though, so he could still sit there with ten grav shields and say he hasn't lost yet.