Well, they could take a look at your criminal record. See if you had any kind of relationship with the victim. Ask you some questions about your whereabouts at the time of the crime, and check out your story if you give one. Same things they'd do to anyone they haven't ruled out -- except, by asking for DNA samples, they might have managed to rule out a few more people. Makes things a bit more efficient while keeping it 100% voluntary.
Even if you do decline to give a DNA sample, I find it hard to believe the police are going to be coming after you at all unless they already have some reason to suspect you -- and they're sure not going to be going to harassment level unless they have very strong reason to believe you're the one. I can only guess that hundreds of people will or have refused to give a sample, and, even if the police wanted to harass them all, I doubt they'd have the manpower to do it.
And, at that point, they'll have to concede there's no real reason to suspect you in particular, and move on. That's just what I said would happen if you're "under investigation" for something you didn't do.
Jeremy