Q: How to uninstall DiskDrill completely?

Follow the developers instructions http://help.cleverfiles.com/how-to-uninstall-disk-drill/

You can only full remove it if you know the directory structure of the Mac OS folders. Some files are added to Library and Applications folder deep down the tree. I suggest you do a web search for useful articles on how to remove application files from OS tree structure. I myself removed Disk Drill app from my OS X but it took me take some advise from an expert since I was unaware of Mac system tree.

Or ......just follow the developers instructions.

Mike's answer is the best one. If you have done manual uninstall or have used App cleaner you will need to install it again and then uninstall again using mike's link.

Yep... that works