The type of echo/reverb effect you're describing would be difficult even for a seasoned pro to eliminate. It's characteristics aren't easy to identify and isolate (versus distinct clicks, pops, hiss, etc., for example). You might try experimenting with some of the various "Noise Reduction" and "Band" filtering effects. Long term, the key is to optimize the live recording scenario as much as possible. Pinning a wireless mic on the speaker's lapel is the best solution but assuming that's not possible, a highly directional shotgun mic should help.