Disconnecting a Git repo from Gerrit does not undo the replication from Gerrit back to Tuleap.
The result is that changes pushed to Gerrit will be replicated over to Tuleap and will provoke Gerrit errors. See example below:
[2013-05-09 12:35:48,045] ERROR com.googlesource.gerrit.plugins.replication.ReplicationQueue : Cannot replicate to
gitolite@tuleap-server:patrick3/u/lmcpare/repo2.git; repository not found
You probably need to revoke read rights to this repo, in Gerrit, to the replication group so that the repos in Gerrit and in Tuleap are really disconnected.