Jews were most certainly banned from the use of organized violence, that privilege only belonged to lords or kings. However, because Jewish merchants loaned money to these most powerful groups, they could sometimes call upon them to force dead beats to pay up.

However, violence wasn't the most common way of encouraging repayment. Because the most prolific lenders worked in cartels with other merchant-bankers, the increase of one's interest on a loan was often incentive enough to get people to pay back their debts. It's worth noting that people taking out loans in the middle ages would not usually be from the poorer segments of society, and if they were, they were often personally known by the lender. More often than not, they were merchants themselves, ship captains, or feudal lords/kings who would need good credit for their future livelihood. They were also usually familiar to the lender and could easily be tracked down due to the local nature of the times.

Escaping one's debt would entail leaving one's life behind completely or being a total dick. This last was usually only available to kings and high lords who would either ignore the Jewish lenders asking for repayment or order action be taken against them as heretics. The latter course often took the form of exile or pogroms where Jewish populations were slaughtered en masse. Such occurrences were not uncommon in the Italian states and France between the 13th and 16th centuries.

I apologize for any typos and lack of concrete detail. I'm typing on my phone on a plane, but this information mostly came from Braudel's Mediterranean vol.2 chapter: Societies where there's a long section on Jews

Edit: I should add that using violence was probably not unheard of for collecting debts, but that its use was significantly curtailed by the anti-Semitism of the middle ages. For a Jew to use violence, especially against a Christian would often mean running the risk of death. Though customs, laws, and sentiments obviously varied from place to place, for a Jew to be engaged in violence was not something most local rulers would permit. This the reliance by Jews on local administrators to dole out the beatings or an increase in one's interest rates

King Edward I exiled all the Jews of England in 1290. This move was roundly supported by his barons, because it basically wiped the debts clean. Jews didn't return to England for hundreds of years. There was basically no Jewish influence in England through the rest of the midieval period.

So, their attempts to collect debts went terribly wrong in that circumstance.

Medieval historian here (specifically medieval Mediterranean). Loaning money was legal in Roman Law, which came to dominate the legal infrastructure of the Mediterranean starting in the 13th century.

Loans were made by Jews to their clients using the existing legal infrastructure; they weren't mafioso in any modern sense. The deals were recorded by notaries in registers that were legally-binding documents and admissible in court. The vast majority of deals went off without a hitch. However, since they were subjects of the King (of Aragon, let's say) or local authority, if someone didn't hold up their end of the bargain (IE, paying them back), they could take up the issue with local authorities, call in the notary to demonstrate that a legal deal was struck, and then the local authorities could/would use whatever legal means or force to ensure payment.

So, to answer your question, they'd just take the defaulted client to court.

Think of it like modern banks. A lot of people hate banks, just like in the Middle Ages a lot of people didn't like the Jews. But that doesn't mean you can just cease payment on a loan. The Middle Ages is less different from the modern era in terms of credit, loan, and exchange, than many people think. Lemme know if you have any questions.

EDIT: obligatory "source": for examples of money lending documents and disputes, see Jean Regne's History of the Jews in Aragon, edited by Yom Tov Assis. For an analysis of the economy of Jews in the Western Mediterranean, see Yom Tov Assis, Jewish Economy in the Medieval Crown of Aragon.