If you have a Domain Server: Open Administrative Tools > Active Directory Users and Computers and find the appropriate user account. Note: You can search for a user account by right-clicking on the domain name in the left pane and clicking “Find…”
Right-click the appropriate user account and click “Disable Account”.

If you have a workgroup server: Open Administrative Tools > Computer Management. Expand Local Users and Groups on the left pane and select the Users folder. On the right pane, right-click the appropriate user account and click Properties. Check-box “Account is disabled” and click OK to apply and exit.