Pop screen is the way to go. I prefer the metal ones over the nylon ones, but they both work. Alos, re-aim the mic so it isn't in the breath blast.
This really is a singer problem. Plosives and "S" problems are normally the singer. It tends to happen with people who don't sing very loud. P's, B's, T's and S's are always about the same volume all the time. If the person is singing quietly, those problem sounds are louder in comparison and become a problem. They are also a consequence of poor breath control.