I agree, if you can open your capx in c2 then deleting the plugin from there is the easiest way.
Outside of the editor if you are using a project folder or if you unzip the capx file to a folder, then you can open the .capproj file and there's a list of plugin's used toward the top.
To remove a plugin manually you can remove that line, and then scroll down to the list of object types using that plugin. From there you can get the object name and then you need to search all the layout and event sheet xml files for occurances to that name and remove it. I've used notepad++ to help finding the name in all the files.
If you miss something or mess something up c2 will give a file and line number which may help. You'd probably want to backup the project beforehand.