Dathorn_Andrew,
There is nothing to stop overselling, but resellers can't use the overselling feature to get more space/bandwidth/features than the Admin has allowed them. For example, a Reseller could be given 100 MySQL databases and then assign 500 databases to each of his customers. However, the second 100 databases are used, no more can be created.
When an account reaches its bandwidth limit, it becomes automatically suspended and the control panel / web server no longer functions for that user.
DirectAdmin has a notification mechanism that will warn users/resellers before this becomes a problem. This way they have time to purchase more bandwidth/space/features from their host before anything happens to their account.
You can suspend individual users. It is under the "List / Modify Accounts" feature in the Reseller panel.
Mark