From the Members list, click a member to view their Member Details page. Click on Settings on the left column to see all the settings available for your member.
On the Permissions section (which is the default), you will see a checkbox marked Suspend Account.
Checking this box will prevent the member from logging in, and will also prevent the member from removing the connected social media accounts (as a security measure). Suspended members do not count against any user caps or limits on your contract.
Note: Members cannot be deleted, as this would impact your reporting and other data.