You need permission to edit users. The Admin role has it.
Go to Settings > Users.
Find the user and select Disable. They're signed out and can't sign in again.
To give them access back, select Enable on the same row.
[Image: The Users list with the Status column and the Disable action]
UDP doesn't delete users. Disabling keeps their history, such as the reports and dashboards they created, and you can enable them again at any time.