[Kentaurus]

It really depends on the hack. Probably you have the readme or instructions that were used to install the hack? You'd need to follow the uninstall instructions (if available), or to at least undo whatever was done to install the hack, meaning:

If you did code edits, comment or delete that part of the code, or reupload the file
If you did template modifications, undo the modifications or revert the template
If you did database modifications...
and added a new column, delete it
and modified a column, check a stock vbulletin installation to modify it
and added new tables, delete them
If you added extra files, you can also delete those
If you change permissions in files (like creating a chmod directory), you can delete that

.... that's for an old-school hack Products have made uninstalling a mod a lot easier