Reverse the setup for your browser. To do this, first refamiliarise yourself with the instructions for setting up your browser to use WebMask, and the explanation of how this works. Then reverse those steps by either:
If you have a proxy set in the WebMask General page, copy this information into your browser as your HTTP proxy.
If you didn't have such a proxy set, configure your browser to use no HTTP proxy.
Delete the directory that you unzipped the program into.
Delete shortcuts to the program (e.g. in startup or HKLM\Software\Microsoft\Windows\CurrentVersion\Run).
Delete all WebMask registry items (all in HKCU\Software\PseudoFoo\WebMask).