It is easy to do - a few button clicks (generally) - but the button location is damn unintuitive. So, this is what you have got to do
1. Go to "Help" menu item.
2. Click on "About ..." button (why on earth should I click that when I am trying to un-install a plugin. By the way, the menu item just above "About ..." is "Install New Software ...". Would it have been too much pain to have a "Manage plugins" and / or "Un-install plugins" right underneath it?)
3. A form opens up. At the bottom of it there is button "Installation details". Click that. (Again, why on earth would anyone think "Installation details" would have anything to do with un-installing stuff. I would have expected only a static display of stuff that are already installed.)
4. Another multi tabbed form opens up (Anyone keeping count of the number of windows opened already. This is the 3rd window by now, including the parent editor window) which shows all the installed plugins.
5. If you select any of the installed plugins, a button to "uninstall" becomes available.
6. Click that and you should be able to un-install and after a restart everything should be fine.
My interest in software and IT has always been much more than a 9 to 5 job (and I am sure there is a huge population that it holds equally true for). I have always wanted software to be efficient and beautiful apart from doing it's job. However, it took an excellent session on usability (which I joined only with casual curiosity but left with renewed interest in the subject and admiration for David Travis who delivered the course) to get me to start looking at all software with an "user's" perspective. And I was surprised with what I found and how it changed my coding.
I have been using Eclipse and STS for years now (nearing a decade now) and I absolutely love these software. However when you start looking at them as a "user" and not only as a developer, there are quite a few usability opportunities of improvement that meets the eye.
This article - apart from helping folks looking to un-install plugins in Eclipse - is also intended at folks who design Eclipse - just a humble request to consider this also as a usability improvement.