How to remove admin error messages

I'm using elgg 1.82 and i tried to use a plugin, but after i disabled the plugin, there was an error message showing for it at the top of the admin area. here is the error message:

river_activity_3C (guid: 47433) cannot start and has been deactivated. Reason: Cannot include start.php for plugin river_activity_3C (guid: 47433) at C:/xamp/htdocs/mod/river_activity_3C.

 

The plugin that created the error is located here http://community.elgg.org/pg/plugins/project/844663/developer/thuvalpakshi/3-column-river-activity

 

I have disabled the plugin, and deleted the plugin folder from the mod folder, and the error still shows. how could i get rid of that particular error?