A custom plugin cannot be deleted if it is used in a container. It may happen that you cannot delete a Tag Integration plugin, even if it is not used in any container. This happens if the plugin has been used in a previous version of a container. Internally Mapp Intelligence keeps the version of the container from before. This prevents deleting the plugin as it has been used in this previous version.
To be able to delete the plugin make a change to the container, reverse the change, and publish it. Then also the previous container version does not contain the plugin anymore. Repeat this with all containers that previously used the plugin.