There is a known issue with versions of Plesk being unable to delete some MSSQL Databases.  Most often, these are DNN databases... the problem is due to the fact that Plesk's "delete database" function attempts to remove the DBusers first, then the DB.  When a MS SQL DB has "objects" created in the DB that are assigned to a DBuser, then you cannot delete the user, the objects (or the DB) has to be deleted first.  It's a little like the old chicken and egg scenario...

(db "objects" are things like views, stored procedures, and roles (we're not talking about records/data).

To resolve this, simply submit a support ticket indicating you're attempting to delete an  MSSQL DB, and the control panel will not allow you.  Please be sure to specify:

1. the name of the domain
2. the name of the database
3. confirm that you would like us to delete the database on your behalf.

And the support team will delete the db for you.