How to completely remove Folx on Mac OS X?
For prior version than Folx 4

Go Download the Folx.dmg from their website, and open it. In the dmg image, there will be a package file called uninstaller. Run that.

It is only for prior versions than Folx 4, as said by Eltima Software. For Folx 4 or newer, it is mentionned on their website that it is enough to remove Folx.app to permanently remove it.