This was exactly my problem, any time I tried to disconnect my Twitter account from Ask.fm there wasn't an option to remove it. I tried everything and nothing worked, but today I found out how to do it.
This happens when you create your Ask.fm account with your Twitter account, using the Login with Twitter option on the start page of Ask.fm.
If you did this, you'll have trouble on removing your Twitter but don't worry, I know exactly how to do it!
The problem is, when you create your Ask.fm account with Twitter, you're not creating a username related to a password, once the password you use is from Twitter. Ok, let's do it...
Step 1: Log in to your Ask.fm account.
Step 2: Go to Settings > Profile
Scroll down and click Change password to create a password for you ask.fm account. This
will allow you to log in to ask.fm without using your Twitter account since now you have a
specific password for ask.fm.
Once you created the password, click Save.
Step 3: Now go to Settings > Services
There should be an option to Remove connection under Twitter. Click there and you're good
to go! Your twitter account will no longer be connected to your ask.fm.