What can be done to stop an elderly person that keeps borrowing money and running up credit cards?

Don't just destroy the credit cards; cancel the account. It also wouldn't hurt to get a credit report to see if he's run up other debts. In addition, if he has bad credit, the only place he could get a loan would likely be a high interest lower class lender, so be alert for something like that to take place.

Do you have any idea though what underlies his lottery ticket purchases? Is he a "get rich quick" kind of guy? Does he have debts or financial obligations that need to be paid off?

If there is a DPOA or POA - AND, depending on how it’s written AND having a doctor confirmation that he is incompetent - a POA might be sufficient.

But more than likely, yes - guardianship would be the route to take.