Are you handy with MS Access, or any programming language that you can send SQL commands towards the database?
For SCM/PCTill versions up to and including 3.6.x, fire in the following SQL
UPDATE Products SET UnitsInStock = 0
(obviously back up your DB beyond doing this!!!)
For SCM/PCTill versions higher than 3.7.0, this will not work. There will be a similarly easy SQL command, though.
Get free EPoS software at http://www.lettronics.com