Under System in WHM's Tweak Settings, there is now this option:
Accounts that can access a cPanel user account
The choices for this option are the following:
# Root, Account-Owner, and cPanel User default
# Account-Owner and cPanel User Only
# cPanel User Only
If you select the first, anyone can log into the cPanel account (root, the reseller owner and the cPanel user). If you select the second option, root cannot log into the cPanel accounts, but the reseller owner and cPanel user can. If you select the third option, only the cPanel account owner can access the cPanel account.