As discussed in phpBB • Signature in posting? the user permission "Can use signature" is not used to check in two places. 

When posting, in the Options tab the "Attach a signature" check box appears even if the user does not have the permission to use a signature. 

Likewise, in UCP => Board preferences => Edit posting defaults shows "Attach my signature by default" even if the user does not have the permission to use a signature. 

In both instances, if the user does not have the "Can use signature" permission, these check boxes should not show to the user.