You can't. Legally, you're still single, & therefore approachable. "I have a boyfriend" is not a legal status recognized by any court in the world. Until you are engaged or married, any single guy can approach you.
Lol! You can not stop them from asking. People are always more attractive when they are unavailable. Just politely turn them down by stating your with someone. You want to keep your options open in case you ever breakup with current boyfriend.