If you can goto your database (via phpmyadmin or some other methods)，you can delete it there, of course you need to be careful deleting the form. You can look into the wp_post table (if you didn’t change the default table prefix), and look for the ID column that has the same value as the form id. And delete it.
But it is strange though you cannot delete the form in the plugin. It might have some plugin conflicts there. You can try deactivate some other plugin and try to delete it.
Actually you can just leave it there if you not using it. It won’t took much your server resources (nearly zero).
