If you set the lead to Black Listed = True, they will never receive content, so this would meet your requirements perfectly. You can set this either by the checkbox in the Lead Info view, or via Lead Database > Lead Actions > Marketing > Change Data Value.
As long as you keep them in the database, even if you import them in again Marketo will merge the lead data automatically so they will remain Black Listed.
I suggest creating some processes concerning how to deal with situations like these and making sure your team are on board with it. It's hard work in the early stages, but worth it in the end!
🙂