@Scott Foster You could go one futher and have a checkbox on the vendor record for blocking transactions and then have a workflow/script that checks that checkbox on entry and flags an alert which you could then redirect back to the previous page they were on.

Thanks for the tip. That's along the lines of what I was thinking, the issue I can see if that a lot of workflows would need to be created

Why loads of workflows? 1 Checkbox 1 Workflow.