You MUTE those with either your fretting hand(by laying just enough pressure on the neck to produce a "thud") or with your plucking hand(use your middle finger to hold/mute the string while your index finger plays the "note").
There's other methods(I'm sure)...
Additionally: If the X is accompanied by a P or T, pop or slap the note respectively, with your right hand while muting the strings with your left. If the X is accompanied by a L or LH, give the fingerboard a slap with your left hand fingers.