The easiest might be to create a dummy form with just an HTML block explaining to the user there is a ban in place and that you aren't accepting submissions at this time. Instead of disabling the process, just update the form in the message start event to the dummy form and use JS to remove the submission button. Or instead of JS, you could use a hidden variable to route the dummy form straight to an end event so any submission of that dummy form just ends (or sends an email to the initiator explaining the ban again and then going to an end event). As an alternative, you could create a hidden required field which would prevent submission all together. 

When you are ready to accept submissions again, just flip the message start event back to the original form. 

Alternatively to all that, if you actually want to disable the process, you can change the starting URL of the main process to something else and create another process again with a dummy form explaining the ban. Set the dummy process to use the main process URL temporarily so anyone with the link sees the notice. When you are ready to reopen the form, change the URL of the dummy process and reset the main process URL back to the original value.