Can you legislate against discrimination?

There's been a lot of focus in the last few months on discrimination: age, sex, race, even weight! New York Times had a feature which touched on this today. Basically, Craigslist is being sued for advertisements appearing on it which say things like, (and I quote from the NYTimes article)

"African Americans and Arabians tend to clash with me so that won't work out,"

and that's apparently against the law -- specifically, the federal Fair Housing Act. The person making the ad can be sued -- and now they're testing to see if Craigslist, carrying the ad, can also be sued.

My question is, if Mr XYZ placing the ad actually did feel that way, how would forcing him to place an ad minus the discriminatory comments, help to give fair housing? In the end, he would have those thoughts in his head when he makes a choice of room-mate.. Unless of course, you're saying that say an African American applied for the room and didn't get it, then he would be able to sue Mr XYZ for discrimination because he didn't get the room! Which would be mind-boggling to me, because, what, if I were letting out a room, I would discriminate actively against smokers, against druggies, against... well, if I just didn't think that I'd enjoy staying with him I wouldn't let him the room would I?

Or, look at this other NYTimes article, on an imam in NY. On page 4, the imam discusses how a tone-deaf man could sue for discrimination if he were not allowed to give the call to prayer. That's like, if people who couldn't sing wanted to lead praise and worship and could sue you if you didn't let them.

Of course, that's robably an extreme view of discrimination legislation in the US. But still, the main thing is that you can't effectively legislate against it. Instead, changing mindsets is the way to go...

Question: Is discrimination bad for the person discriminating, relative to the rest of society? If it is, we shouldn't need to bother, those guys can figure out what to do to help themselves. So for instance, if people refused to do business with people who discriminate, then the practice would soon die out.

On the other hand, if discrimination imposes costs on society (probably true, though I won't argue that at length today), but doesn't hurt the person discriminating: then there's no reason for him to stop discriminating. It's like the negative externality of traffic congestion.. no individual person has enough of incentive to stop driving, because most of the costs he inflicts is on others.

So if you want to end discrimination: discriminate against those who discriminate, today!