You can delete your snapshots from your Memstore Account using the Manage Snapshots feature within your Memset Control Panel.

To do so navigate to your Cloud Storage section in your Memset Control Panel, select the Cloud Storage Product and select "Manage", from within the Management page, locate the "Cloud VPS Snapshots" under the "Storage Tools" section, you should see a page like this;

You can delete a snapshot from that table by clicking on the red cross next to the Cloud VPS and Date of the snapshot (this action can't be reversed), this will then remove the snapshot from your Memstore.

Snapshots should only be deleted by using this method, attempting to delete them from FTP/SFTP may cause you issues resulting in part of the snapshot remaining and still consuming space within your Memstore.