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is it possible for Marriott hotels to forbid the use of personal hotspots? Part of the ToS guests have to sign?

You'd have to check through ALL FCC rules. They can preempt and nullify any such agreements or rules that affect wireless device use. They've really put their foot down for OTA TV, DBS (satellite), and WPS (formerly: wireless cable TV), and could do so for WiFi:

http://www.fcc.gov/guides/over-air-reception-devices-rule

You'd need a lawyer specializing in this stuff to determine if they've made any rules that might apply to restrictions on the use of WiFi.

Ok, being able to send "de-authorization" packets does not mean to be able to identify or localize the hotspot.

It's extremely easy to locate a WiFi hotspot. Android devices have WiFi Analyzer which will beep like a signal meter as you approach a given AP. Then just walking around the location, you'll be able to use that info to narrow it down to a 20ft area, or so. You can do the same with any WiFi device that displays the signal strength of individual APs, just needing to watch the numbers, or otherwise write your own program to beep and show a relative gauge.

It would be much faster, easier and more accurate still, if coupled with a directional WiFi antenna connected to your device, to narrow it down to exactly which person at a table has the AP in their pocket.