1 - Deactivate unneeded plugin

2 - Activate the bundled plugind Garbage collector and Log rotate and set 'week(-ly)' options (cron must be installed)

3 - Remove the plugin's folder from '/mod' directory in your Elgg root.

4 - Run flush the caches.

Cron job will clean the unused DB records during next event trigger cron job.

Sorry to say. But I don't think that all plugin data will get removed by Garbage collector. I would even say most of the data won't get removed. Elgg does not remove valid data as you might decide to re-enable the plugin again at a later time (it would be quite annoying if you disable a plugin for some testing reason and then the garbagecollector cronjob would run and all data of the plugin would be gone when enabling the plugin again).

Plugin settings won't get removed. Data saved as entities or annotations won't get removed either. The garbage collector process would only remove orphaned metastrings and some other orphaned data that are not immediatelly removed when the entites they belong to are deleted.

If you want to clean up your database from all plugin settings etc. you would have to write your own script for the deletion specifically for the plugin you want to remove the data from. But the settings data shouldn't be that much to bother about. It might be different with content data created with a plugin. But you would have to write a delete script for deletion of content data, too, if it's too time-consuming to delete the content entries via the delete button on site.

Yep. Forgot about plugin settings. Thanks for clarify it.

BTW, there's https://github.com/ColdTrick/garbagecollector_extended