You can disable a User's account by going to their profile in Channeltivity and clicking the Actions button in the top right corner of the page > Disable. 

You can also disable Users in bulk by going to Admin > Users > select the checkbox next to the Users that you want to disable > Actions > Disable. 

What happens when you disable a User's account? 

The User will no longer be able to log into the portal, receive Leads, register Deals, etc. 

Who can disable User accounts? 

System Admins can disable User accounts, and they can also give Primary Contacts of Partner and Distributor Organizations the ability to disable and re-enable User accounts by going to Settings > Portal Configuration > General Portal Settings. Scroll down to the User & Organization Profile Settings and select the "Allow the Primary Contact of a Partner or Distributor Organization to disable enable other User within their Organization" checkbox and click Save. 

How do I re-enable disabled User accounts?

You can re-enable disabled User accounts by going to Admin > Users and selecting the checkbox next to the names of the Users you want to re-enable > Actions > Enable.  

What happens when you re-enable a disabled User account? 

Re-enabling a disabled User account allows the User to log into the system again.

Related Documentation: 

What do the different User Statuses Mean?

Users and Contacts

How do I delete a User?