We don't recommend to delete the plugin folder from your server without first deactivating the plugin:
- you will lose all data associated with the plugin. For example if you delete "Photo" plugin, you will lose users' photos.
- just deleting folder with the plugin is not enough. You will need to clean up the database and some other files where this plugin's files are stored. In case you do something wrong, you will be getting even more errors on your site.
The best solution is to deactivate the plugin via the Admin Area.
Only in case you can't access the Admin Area because of the error, you may delete plugin manually via the database.
Here you may find some instructions how to delete plugin manually:
http://www.oxwall.org/forum/topic/12849?page=1#post-64903