Is it possible to force someone to unfollow you on Twitter? Use-case: I want to follow my ex-girlfriend, but I don't want her to follow me. If I block her, I guess I can't see her tweets. Also, I would rather not use any 3rd party tools like follow her on RSS and block her.
No you cannot. Twitter is an application for sharing. She has a right to follow anyone she wants if the person allows them to. Blocking is the only way to technically prevent her from following you.
Here is a suggestion of what you can do instead.
You can change your settings to protect your tweets
Only let people whom I approve follow my tweets. If this is checked, your future tweets will not be available publicly. Tweets posted previously may still be publicly visible in some places.
Then add her to a list. Allowing for a quick link to her profile.
You can block someone and still view their tweets so long as they don't block you back but you have to manually go to their profile and click view tweets each time. That's really the only sure-fire way to keep her from following you while still seeing her tweets (which I assume is the reason you would want to continue following her) or you could create a second account and just follow her on that