If a certain visitor or user of your site is causing you trouble you can attempt to block their IP Address to prevent them from accessing your site.

To do this you first need their IP Address, which you can find by going to your Site Admin and then the Audit Logs tab and find an action they performed, as their IP is logged with each action.  Then go to the Block IP tab of your Site Admin and click the "Block an IP" button which presents you with the following popup:



Here you can enter their IP Address along with a reason for blocking them, and upon submitting the form their IP will be blocked.