How to migrate down database?
You have to migrate down BEFORE deleting a plugin. Time to pray that you haven't deleted the migration added by the plugin...
The syntax required is:
rake db:migrate VERSION=XY # where XY is the number of the latest migration you want to keep. Have a look in db/migrations/ directory to decide what the good migration is.
Try to provide detailed information if this is not enough (redmine version, plugin name installed, in which version, etc.).