If you can't block the e-mails at work, just don't read them. I'd look into identifying them as spam so they go into your spam box instead of your inbox. Do you have an IT department that could block the e-mails?
You are right not to pay in cash. You not only don't want to see him but you want proof of payment. Do banks there issue certified checks? If not, I like your idea of sending a check. You might want to make a post on the legal board to see if there is something else you can do to protect yourself - maybe some wording as to full and final payment, for example.
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