if you hover over a share, you will see a settings wheel on the right. Click on that, and select "recover". That will give you a pop-up which (after a while) should show you all the snapshots for the share. These all have date names.
You can select more than one by holding down the shift key. Then right-click and you will have a "delete" option. Delete the oldest first.
It might take a while for the space to be reclaimed. After the snapshots have all been deleted you should run a balance, and when that is done you should also run a defrag.
If the snapshots don't show up in "recover" or if the space is not reclaimed you might need to contact support.
BTW if you click on the settings wheel for the share, and choose "settings" there is a snapshot tab that gives you controls over snapshot use. Custom snapshots lets you select a retention period (after which the snapshots are deleted).