TempDB will not shrink. No Open Transactions
Note: this post might be useful too:

Issues with TempDB mdf file ever increasing

Unless you can figure out what process is using that work table (and can safely kill it), I'd have to agree with what your searches have already yielded: cycle the server and you should be able to shrink tempdb.

A different question has dealt with figuring this out for #temp tables; I don't know if it can be adapted for work tables:

Find which session is holding which temporary table

I've also blogged about it (again, for #temp tables):

http://sqlperformance.com/2014/05/t-sql-queries/dude-who-owns-that-temp-table

I doubt the work table is related to snapshot isolation / version store, but just in case:

Find transactions that are filling up the version store

Also, don't rely on DBCC OPENTRAN; - I've observed many scenarios where I know I have an active transaction but it doesn't show up there. And note that database context is important; the database where the transaction is active is not necessarily tempdb. What do you see here? Anything?

SELECT * FROM sys.dm_tran_active_transactions
  WHERE name = N'worktable';


Once you've shrunk tempdb

Of course, this isn't a permanent solution. You're going to shrink tempdb, and then it's going to grow again. This can become very boring and tedious to play this game every time it happens. And if it is going to grow again, what are you going to do with that free space in the meantime? Lease it out and then evict people when tempdb needs it again? You need to either:


Fix the process that is making tempdb grow abnormally large in the first place.
Allocate enough space for tempdb so that it doesn't need to grow, and stop shrinking it (especially if only temporarily; this is just wasted work!).


A couple of other suggestions:


Don't use SHRINKDATABASE (which should be called auto-fragment) or the UI. Write specific, targeted SHRINKFILE commands to affect individual files.
Consider using multiple files for tempdb (which you could spread out to different storage if/when possible), and consider trace flags 1117 (as long as this behavior will not affect your user databases too) and 1118. 
Some tips on minimizing tempdb utilization here: 


How to identify which query is filling up the tempdb transaction log?