Default group is able to use stat track even though the permission was never given. 
I also tried revoking stattrack.use perm from default group and just get the message danault group doesnt have permission stattrack.use.

Disregard. I believe it was just a player finding a weapon from a group that does have the permission.

It can't be, because there are check for it. Be sure that you removed it from all groups you have