I don't know if this is possible or not, but I've got audio from recording a conference where the sound out of the PA has been captured by the mic on the presenter, creating a harsh echo type noise on their voice. Have a listen to this sample I placed on Google Drive:

https://drive.google.com/file/d/0B1AvxrsaFfdeb05ualJrOVE3LTQ/

Is there any way to use say, Adobe Audition to fix it? If not, is there a way to prevent it in the future?

I don't use Audition so I won't guess but honestly it didn't sound as bad as I thought it would.
Maybe throw a touch of low end on the EQ into your audio and perhaps that'll help.

At seminars, conferences etc of which I do a lot, I pull a feed from the sound board but I'm also usually working with a sound guy that I know and have worked with before, so I know what I'm going to get AND I always monitor with my headphones. Barring that, you need to mic the speaker up directly back to your camera with YOUR own lav set up IF you can.

The iZotope RX3 advanced has a 'Dereverb' tool you could try. RX3 advanced ain't dirt cheap though. Don't know if the demo version is full functioning.

...is there a way to prevent it in the future?

Mostly, yes. Signal to noise ratio in this situation is about the distance between your mic and the speaker's mouth. The closer you can get, the less trouble you'll have with first reflections (or unruly PA speakers). It's not that the reflections won't be there -- they will be. But the speaker's voice will now be much louder than the reflections, which pushes the reflections into the noise floor, which is where you want them. Lavalier mics are popular for a reason. If you can't put a lav on the speaker you can try getting a line level out from the PA system. If you can't do that, maybe you can put your own mic on the speaker's podium. Specifics don't matter -- you probably understand what I'm telling you by now, which is: get close.

@Rick

Right on - Izotope RX3 Advanced ain't cheap. But it's one of my all time favorite programs. Well worth the $$$ (or should it be $$$$$) IMHO.

If you have Audition CS6, you might try effects >adaptive noise reduction > derverb single source and play with the controls as you listen, stop before it gets "chirpy." A noise sample is not necessary with this plugin.
It probably can be improved upon, but attached is a sample: