***EDIT*** There is now a way to do this: (from Booklog Tech)
If you are looking to clear out the pre-auth or release the funds stuck in Payware - the best way to do that is to Log into your payware console: https://prod1.ipcharge2.com/mc/
1.) Go to Transaction Processing and choose Web Terminal.
2.) Choose Completion from the radio buttons across the top.
3.) A screen of all of your pre-auths will open.
4.) Enter the transaction id of the one that you want to void.
5.) Enter a trans amount of 0.00.
6.) Press enter on your keyboard.
7.) The transaction will come up as completed/voided
The PayWare Developer Documentation actually states that there is no way to void a Pre-Auth using their API (from a Website).
However, after a little digging around, we found that from your Virtual Terminal, you can locate a Pre-Auth transaction and void it. (We believe this will release the hold, but are not positive.) You can find it here in your PayWare virtual terminal: Transaction Processing >> Void Reprint Search
If you are able to succesfully employ this method, please reply, and let us know.