This is highly dependent on how exactly it was installed. You would want to delete any Wordpress files. If those files are in a specific folder, it would be a simple as deleting that folder. If you mixed it in with IPB files, you would have to sort out what is which and only delete the Wordpress files. If you don't know which is which, you can download a set of IPB files from your client area and look at what directories, files, etc are associated with IPB.
If your Wordpress is installed in a separate database, you could simply delete that database. If it's shared with your IPB install, you would need to manually separate those as well.
If you had any 3rd party integrations to tie your Wordpress to your IPB, you would want to remove those as well.