How do you restore a DB to Multi-User mode when deadlocked by a sys process
Apart from what Shawan referenced, another option is to use DAC ADMIN:ServerName
then issue below command (note that I am not using NO_WAIT)
ALTER DATABASE database SET MULTI_USER with rollback immediate

From BOL :

ROLLBACK AFTER integer [SECONDS] | ROLLBACK IMMEDIATE
Specifies whether to roll back after the specified number of seconds or immediately.
NO_WAIT
Specifies that if the requested database state or option change cannot complete immediately without waiting for transactions to commit or roll back on their own, the request will fail.