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Theoretically, when you have a business relationship with someone, you and the business can sign a contract together containing anything that is not against criminal law. "You agree not to use your personal hotspot while you are in our hotel" is not against criminal law. The enforcement of that contract would get bad, though - they obviously can't block other service, they just got fined for that. They would have to proactively search for wifi other than their own within the building, address the person serving it, and request that they either stop or leave the building. (Technically speaking, that's quite easy. Just look for the radio waves.)

If that person had signed the agreement, they would then be liable under the agreement and could be penalized or sued or some other appropriate punishment. If the person had not signed the agreement (and keep in mind, they might not be the ones who made the travel arrangements and things would get trickier there - just because you authorize your travel agent to reserve a room for you does not mean you authorize them to sign legal agreements on your behalf), the Marriott could only evict them, and request police assistance if they refused to leave.

If the case went to court, the victim's contract lawyer would have a field day confirming whether or not the contract was actually valid - any number of factors could get it ruled invalid. If the victim was not the one who made the arrangements and did not authorize the signing; if the victim was in any way incapacitated; if the victim was presented with "oh, it's just a standard form, you have to sign it" talk by the clerk; if the Marriott failed to uphold its end of the contract in any way, including not providing the mandatory complimentary breakfast with hot oatmeal as specified in the contract; if the Marriott selectively enforced the rule (HE got away with it, why are you picking on ME?)... the list probably goes on quite a while, but IANAL.

The victim could then hit the hotel up for attorney's fees and infliction of emotional distress, and might do so anyway, regardless of whether they won or lost the suit, arguing that the agreement was egregious compared to the agreements other hotels force you to sign, and was deceptively pitched - and they very well might win. (It'd probably end up as a class action suit for all guests who had to sign the thing.) The lawyers would probably have a field day and everyone else would get shafted, as is usual for these things.

But, yeah, that would get rid of those "customer" critters Marriott would rather like to keep getting paid by, so you're right, it's only theoretical. Theory is fun!