Hands OFF! 2.3.4 not able to install on 10.10.3

I had this problem also and here's the solution:

Get into the Library folder (CMD+shift+g) and make a search "handsoff". You'll find one folder with the old installation files inside and a file called "handsoff.kext". Just delete those 2, reboot your mac and then you'll be able to install handsoff again.

Have to uninstalled by hand ?

To uninstall Hands Off! go in the Preferences window and under the Advanced tab, click on the Uninstall button. Then, simply move the application icon to the Trash.

That's the recommended step by One Periodic

I think he uninstalled it manually. When uninstalling it manually, certain files at the library folder wont be deleted and needed to be deleted manually before trying to install a newer version.