In golf tournaments, what disqualifies golfers from completing rounds?

There are a lot of things that can DQ a golfer, check a USGA rule book. Too many clubs, illegal or illegally modified clubs are probably the most common. You are also allotted a limited number of to complete a round, I forget the number, see Tin Cup. If you lose a dozen, technically you can't sign your card. There are plenty of other rules as well, see the book.