The security roles and tasks pretty much give a user access to a window or not, it doesn't limit the functionality within the window, which is why you're not seeing any security operation that would allow/prevent a user from being able to create a new GL account.
What you'll want to look at, is more Field Level Security, which can be used to disable buttons and fields within a GP window, that could potentially allow the user to have access to the windows they need, but keep them from being able to create new Gl accounts.
Another option would be to remove access from the Cards windows for the user, and instead give them access to Inquiry windows where they could view the account information but there wouldn't be any functionality to create new GL accounts.
If FLS security doesn't work, then the next option would be looking at Modifier and/or VBA on the window.
Thanks
