1. Open a Finder window
2. Hit CMD + SHIFT + G to open the Go To panel
3. Enter the path ~/Library/Preferences/ into the input field
You should see a large number of .plist files in that location
4. To find the Preferences file you need, search for the name of the Topaz Labs application you want in the search field. Make sure you are searching the Preferences folder you are currently inside. The preferences file will be named something like this: com.com-topazlabs.Topaz A.I. Gigapixel.plist
4. Once you have found your plist file. CNTRL + Click or right click and select Move to Trash to delete it.