You should be allowed to make simple account closures (to start the process of the deleting of your data, though some will be held for x years) but oddly some casinos insist this is something that isn't possible and, oddly as i found, will try and force you down the route of Self-Exclusion
Agree with Dream in that there have been a few i've simply left open to rot away on their own accord as they took that stance and, as they were part of a group and i played at others in that group, i wasn't convinced they would do it properly (ie they would put down as a Self Exclusion when you type in bold, NOT SE reasons etc)
Even some that have the 'a/c closure' button on the site, once you click it only give you the options for SE/TAB