The best solution I can think of is set up a new instance from a backup 
That’s the cleanest you will get it whilst keeping your forum data without a lot of manual effort.
However, I believe backups will include all the tables that existed in the instance you backed up from and that may include any created by plugins, even if you have not installed the plugin on the new instance.
If you want to get your hands dirty, you can use the rake tasks to drop a database and create a new one and run migrations all over, but that will delete all your data.