See: http://www.apiman.io/latest/user-guide.html#_plugins

3.8.4.2. Deleting Plugins

At any time you may choose to delete a plugin. Note that if the plugin was contributing one or more policies to apiman, then the policy will no longer be available for use when configuring your Plans, APIs, and Client Apps. However, if the policy is already in use by one of these entities, it will continue to work. In other words, deleting a plugin only removes the policy for use by new entities, it does not break existing usages.

To delete a plugin, simply click the "Delete" action for the appropriate plugin on the "Installed Plugins" tab. After confirming the delete, the plugin should disappear from the list.