Let's review those apps and sites which are linked to Twitter. In some cases, these apps/sites post for us (instagram, facebook, etc.), in others they collect analytics about our networks, and much more. They may access our posts, our networks, our contact info -- so always a good idea to review yearly (at least!)
Under Settings (the wheel icon)
Click on Apps.
Click on Revoke Access to the one you want to remove (note: if you've setup a separate account with the app or site and you don't want to use the service AT ALL vs. just unlinking from Twitter you will need to separately remove your account from the site via their website or app, e.g., I don't want to use Instagram anymore - ha! yeah, right - so I would revoke access via Twitter but also delete my IG account and remove the app from my phone).
Twitter keeps the list of apps, so you can always Unrevoke Access later if you want.
More here at Twitter Apps.