The correct really is you ask the developer because he knows exactly what needs to be done with the module.
But, by default, if it is NOT a module BoonEx, the steps would be:
1. If the module appears as already installed, select it and then click Uninstall.
2. Go to the modules folder and delete only the folder of this module. The same folder that you uploaded when you tried to install. You have to do this via an FTP program.
3. After that, reload page again modules and see if it still appears as a module installed, uninstalled. The correct would no longer be appearing in any of the lists.
However, I insist that the ideal is the developer fix this because there can be tables in the database that were created by the module and were not uninstalled in the process above.
Good luck!