To uninstall MantisBT:
Delete the MantisBT directory and all files and subdirectories.
Drop all MantisBT tables from the database, these can be identified by the configured prefix for the installation. The default prefix is 'mantis'.
Remove any customizations or additions that you may have made.
If you have the permissions to create/drop databases and you have a specific database for MantisBT that does not contain any other data, you can drop the whole database.