If you are using ClassLink Sync with auto updates, this is done for you automatically with each daily sync.
If you are using CSV Sync, when you upload your latest CSV student list, students in CritterCoin but not in your CSV will be deleted.
Otherwise you can delete students in bulk with the following steps...
1) Sign into the CritterCoin Admin portal
2) Click the Students side menu option
3) Click the Delete button in the top menu
4) Paste in a list of students emails, one per line, for each student that you want to delete
5) Click Next and confirm