Cannot delete/close open activities

I could not see how to delete open activities, so I closed some of them and then deleted them from the closed activities log. Each time I did this Zoho asked me if I was sure. Affter a few it offered to stop dialogs for that screen and I thought this meant it could stop asking me if I was sure so I selected yes (or OK or something) . Since then, I cannot close any more open tasks so I have no way of deleting them. Can anyone help?