What's the difference between that and the current reality? Food stamps can be sold and bought on the black market in exchange for drugs and alcohol; people who want whiskey will be able to get it. Whatever money is spent on enforcing the "sanctity" of food stamps could just go to more people directly.
And this is really the main point: the administrative burden all of these social programs incur, especially in terms of labor and office space, means that a very large sum of money is going into these programs that will never be seen by the people who need help. Just give everyone a check and be done with it.
To answer your main question: the person who blows their UBI suffers. They will be hungry for the rest of the month, and not many people will have sympathy for them because they already got their check. All mentally sane people realize this reality, and that is enough to prevent them from spending their UBI poorly. For the minority of people who are sadly physically addicted to drugs, there can be social programs specifically aimed at recovery and rehabilitation. This was another one of Yang's policies. UBI would help here too: let's say you spend 8 months at a government funded rehab facility while you kick your addiction to meth or heroine. At the end of the 8 months, you have $8000 waiting for you - to go forward, get a job, rent an apartment, and rebuild your life.
This is much cheaper than a bloated social program that ties your aid to measurable checkpoints, like how you have to prove you're looking for a job in order to keep receiving unemployment. Under UBI, the Fed just runs some script every month to raise a double somewhere by $1000; under unemployment they have to hire people to process your application, approve/deny it, review your weekly application submissions, hire someone else to distribute the money, hire a manager for all these people, pay an office to give these people a workplace, pay pensions and benefits for all these employees, etc.