In CA PPM 15.1, you can lock users in Administration -> Resources screen by selecting the checkboxes and clicking the Lock button. You can then unlock the users by filtering for Status of Lock, and selecting the checkboxes and clicking the Activate button.
Depending on the number of users, this may be an effective solution. You are limited to doing 20 users at once, so it may be time consuming for a large number of users. If you have more than one administrator, you can divide and conquer by having one start at the begining of the alphabet, another start in the middle, then another start at the end.
You will need to be sure not to lock out the administrator's accounts.