If I start getting inundated with stupid meme type stuff, I'll look at that individual's page and see how much of it is junk. If too much stuff is simply low quality reposts, I just block them. No point unfriending them, I still want them to see the stuff I post, but I don't need to see their stuff.
Most of the people left post their own stuff at least occasionally.
Also, you can block the source, rather than the friend. For instance, that might have been a share from a person or group, and your friend simply reshared it. By clicking in the upper right hand of the post you will see an option to block your friend, and if it's a share from someone else, you can block that.
There are a lot of things I've blocked - for instance a number of political groups that post nothing but incendiary stuff. Most of the stupid facebook quiz sites are blocked. A few of my friends seem to enjoy risque material I don't want on my facebook feed, so I've blocked a number of those.
That way I can still see their other inane posts about family and so forth. and if they do have their own political opinion, I'll see that, which is what I want. I'm not interested in thoughtless reposts of other people's work, just what my friends and family themselves are saying.
As far as that post, I'd say nothing. If I really felt the urge to comment on it and couldn't restrain myself, then I'd research the issue so I could do so from a position of knowledge. What exactly do these people think President Obama did that was illegal? What was his real authority? If you're going to bother to debunk something, be prepared.
But honestly it's not worth it. They didn't post it in order to have a discussion, they posted it in order to get your gall, and apparently it's working.