All they can really do is go around trolling other users with public photos and comments. But, it can be confusing to the recipient to be attacked by other "conservatives". It's also time consuming to clean out your inbox and remove comments from photos, videos, etc...
The best you can do is block and report for harassment, especially when they go into hate speech, which they invariably do.
What can they hope to accomplish by posing as conservatives? I can think of a couple things:
- discourage people from publicly supporting Rick Perry or John Cornyn on Facebook
- put the Tea Party in a bad light. They make some really awful comments.
- plant the idea of dissent amongst conservatives
- keep an eye on what people are saying on the "other side"