I suppose I'm more concerned with not appearing to be a bitch than I am with getting my point across clearly. I don't know where the middle ground is.

Part of the trick is not letting the other person's perception of you affect your behavior. I'm learning this the hard way with my ex wife. Just because she thinks I'm an asshole, or treats me like I'm being an asshole, doesn't mean I'm actually an asshole. On one end of the spectrum, it's manipulation, on the other end it's immaturity/insecurity.

If you let the guy down firmly but clearly, giving him the chance to spare his pride, and he fails to take that chance, then forget it. No longer your problem.

Being firm and clear is difficult, but not bitchy. Not all rejected people can see that clearly though. Again, not your problem.

Being direct in the overall result and vague in the details at the same time is how I choose to do it now. And yes, for the first time in my life I have the opprotunity to reject people. In your case I'd recommend something like 'this isn't working for me anymore, sorry but it's over.' And just flat out ignore any pleas for reasons. Assert your right to not talk about it, which will only serve to reassure the person that they don't have another shot, or there is no hole through which they can possibly 'save' the relationship.

Once you've done that, there's little you can do to prevent him thinking you're a bitch if that's what he's going to think. You can however rest assured that you're probably the nicest person who has ever dumped him, because you didn't leave him languishing in false hope or confusion until he figured it out.