If you suspect a user is not acting in good faith, you can lock that user out of their account.
What does locking an account do?
Locking a user's account will immediately log the user out of their account and prevent them from signing back in. If they try to sign in, they will be met with a notice telling them their account has been locked:
A user with a locked account will no longer be able to access the site from that account unless a site manager unlocks their account. Additionally, their account state in the user report will change from Activated to Locked.
How to lock an account
1. Go to Manage > Management Dashboard and select your network
2. After selecting your network, click the Members tab
3. Use the column filters to find your user
4. Click the arrow next to the user's email address and select Edit Member
5. On the Edit Member page, click More Actions and select Lock User Account
6. Enter a reason for locking the account, and hit OK to lock the user out of their account
How to unlock an account
To unlock a locked user's account, go to the Edit Member page and click the blue Unlock Account button to reinstate their account.