Just to add: everybody can always extract the original WIS installation script file from a WPI archive. It's ASCII plain text and the syntax well explained in the documentation that comes with WarpIn.
That is, even for unmaintained WPI packages:
1) we can fix the installation by extracting the original WIS script from the WPI, modifying it to our liking and putting it back into the WPI
2) we can deduce an uninstallation script (Rich's suggestion) from what the original WIS installation script does
The only thing that has always been a mess is that with WarpIn you can easily screw up your config.sys to a point where after uninstallation of a WPI the system will no longer boot. See the FAT32 WPI.
Short rule: NEVER EVER have a WPI script modify PATH or LIBPATH with subdirectory paths that you can expect to exist (like \OS2 or \OS2\DLL).