There is a little bug in Cognos series 7 which can cause this problem. Is your temp db file filling up as well? When you kick off the report, a connection is made to your database (I'm guessing you're using SQL server) and when you click "cancel", the connection is not severed. To correct the problem, you will need to cut the connection at through the server. My understanding is that this bug is resolved in Cognos 8.