The same thing that happens now when people who can't manage their money blow their salary on other things, except that it's marginally clearer what the problem is.

Right now those people get food stamps and welfare

Wow, talk about living in a bubble. Do you really think that a person who runs out of money can just go and get more from the government?

My sister gets food stamps. Food stamps are for food (generally) where as money is for anything. UBI is often held up by proponents as a replacement for all other support. Given them $1k a month cash instead of whatever they get now. But what they get now is trying to solve a problem not solved by UBI, namely that they can't be responsible for themselves (for whatever reason, physically unable, mentally unable, kids, etc...) UBI doesn't fix that problem so the existing programs will still need to exist.

Anyone unable to be responsible can receive their UBI via a cashless welfare card (https://en.wikipedia.org/wiki/Cashless_Welfare_Card) which quarantines where the money can be spent. It deals with issues like problem gamblers and people with substance abuse problems.

hahahah what? Those are directly based on salary, not on "salary after you spend most of it". How would you even implement a system like that?

You'd dole out UBI biweekly or whatever to solve the same problems that food stamps solve? I agree it's a good question though.

One proposal I've heard for this is spreading it over the year, have the government deposit money in a back account once per week (this requires a system where everyone has bank accounts to get UBI, but that shouldn't be too costly).

There are already benefit programs that distribute money by putting it on a debit card, eliminating the need for a bank account.

Doesn't a debit card require a bank account, at least behind the scenes?

It doesn't require the person who gets the card to have, or be eligible for, a bank account. You don't need a bank account to use a pre-paid gift card either.

I suppose they would have an account number within the organization that is distributing the money, but that's not quite the same thing, and surely has less overhead.