Aside from re-shooting it with a proper mic, there ain't much you can do.
You could try.. EQ'ing out some of the room's resonant frequencies, but it will never sound good. Sorry.
I'm with rraud on this. I assume you are talking about the built-in mic and not
a short shotgun on pro-sumer cameras.
In general get the mic off the camera and closer to the talent.
Choose the correct pickup pattern for the task.
A small room's feel can be modified using studio foam (such as Auralex.com Lenrd) on light stands. They can be to an extent portable where a studio is not practical.
no good fix for this.
you wanted to film a horse, and filmed ducks in a pond.
you wanted to record a person, but recorded a room.
you need to reshoot or rerecord the person talking :)
Of the two common approaches, I prefer the "lav" (either wired or unwired) over the "short shotgun" approach -- the reason? It's a room! Do a search on these forums if you don't understand what this implies in terms of early reflection and phase anomolies.
Even outdoors, the lav is preferable for a really good reason: The sound is truly localized rather than focused. IOW, your shotgun not only picks up the speaker, but also everything in a circle behind the person (like the freeway).
Why do you think experienced broadcast teams use those wired Sony 77 lavs both in the studio and on location rather than on-camera mics of whatever configuration? Well, they will probably tell you it's so you can hear the speaker.