You should be able to shrink the log file for tempdb just as you would normally as any other database:
DBCC SHRINKFILE ('templog', 2048)
If you have tried this and it won't shrink, you could run queries to see exactly what is filling it. For your immediate concern, space, you may need to clear the cache first - DBCC FREEPROCCACHE GO (warning if this is production) as this clears the procedure cache and will cause re-compiles, but, sometimes, this is the only thing which makes it work. From BOL - Removes all elements from the plan cache, removes a specific plan from the plan cache by specifying a plan handle or SQL handle, or removes all cache entries associated with a specified resource pool.