A location posts an ephemeral key which people at the location use to prove they are at the location.
For example, a lecture accepts questions from the audience through Twitter, using a new, unique hashtag specifically for this lecture and posted only in the lecture hall.
A band or DJ gets responses from the audience about how well they like the current song. A display posts two live-updating QR codes for websites corresponding to "this song rocks!" and "sucks".
Could probably involve ephemeral private keys given at a location to sign messages from the location.
Create UIs which make location-proved interaction easy for the user.
Avoiding spam is probably the main motivation, but other, perhaps better, spam avoidance techniques already exist.
Proving location can probably never be made completely unforgeable: the key could easily be transmitted to someone outside.