You need to restart your python session to release the cursor, I do not believe there is any other way to delete the cursor and release the lock. If you are running a script tool that is running in-process, this means you need to shut down the app (for example, ArcMap) entirely and start up again.
This is one of the downsides of the old pre 10.x cursors, you had to be very careful to have your python code delete your cursor, usually with a try/except/finally.
arcpy.da cursors are much better in this way because the with construct is usually used -- if you drop out of the with for any reason, the cursor is deleted by Python for you (since you have left variable scope).