You're just going to need to wait, unless you want to risk corrupting other databases by, say, yanking the power cord out of the wall. You can set your other user databases offline before attempting such a stunt, but good luck with system databases.
For a rollback you should be able to guesstimate how much longer you'll need to wait by running this query:
[est_finish] = DATEADD(MILLISECOND,[estimated_completion_time],CURRENT_TIMESTAMP)
WHERE command LIKE N'%ROLLBACK'; -- could be KILLED/ROLLBACK
I blogged about this here (and it includes a much more exhaustive query):
If you know what operation is rolling back, you may also be able to infer some guesses on progress by looking at the columns
logical_reads - but don't spend too much time on that, since I/O is what
percent_complete is based on, so you'll probably come to roughly the same conclusion.
Still, this is just information that will help you set expectations about how long you need to wait. There isn't a very safe way to cancel a rollback.