What consequences are there if, having already graduated and received my degree, I do not pay up my tuition fees?
Some universities (perhaps most) will refuse to provide transcripts for students who owe money. Example.

This can be a serious problem for you. Some employers insist upon an official transcript as a condition of employment. Your  diploma or similar certificate  will not suffice; the  employer wants a document directly from the university, which has not passed through your hands (to avoid the risk of you falsifying it). If the university won't send a transcript because you didn't pay them, too bad, no job for you.

Also, you say you are done with academia, but if you should change your mind someday: US  graduate programs generally require an official transcript from your undergraduate university as a condition of admission. So no grad school for you, either.

Academic matters aside, the university also has all the same options that would be available to any other creditor. This side of things is more on topic for our sibling site, http://money.stackexchange.com, but as a partial overview:


They can report you to a credit agency, which will damage your credit score and make it difficult or impossible for you to get credit cards, car loans, mortgages, etc. Bad credit can also keep you from getting a job in certain industries, especially defense and other work that requires a security clearance.
They can refer your account to a collection agency, who will try pretty hard to bother you until you pay.
In principle, they could sue you, and they'd almost certainly win, as they can easily prove you owe the money. This would enable them to get a court order to seize your assets (eg take money directly from your bank account) or garnish your wages (take money directly from your paycheck).