Nobody 'guards' Tacko Fall. There were games last year where he'd match up with guys a foot shorter than him. All teams try to do is front the post and send help from the weak side in the event that he catches it. His height might benefit him in certain situations, but there is a reason receivers in the NFL aren't 6'10. There is a degree of jockeying that goes into the fight for post position that becomes very difficult when you're one of the largest human beings on the earth.
In time, his ability to time his leaps over the front defender and proceed towards the basket - either to make a pass or to score - in one motion will determine how good he can become. That's what makes him fascinating, though - we're in the same boat as everybody else in the country in that we simply don't match up. In the meantime,
@Dream Jobbed 2.0 's idea of Williams fronting him makes a lot of sense. I'm more worried about keeping him off the glass.