SQL-Server Custom Database Role

If I had to guess you have them added to the db_owner role as well. Either that or sysadmin (although less likely). Administrative roles, specifically DBO and SYSADMIN can not be blocked from access to anything they have control over (dbo - database level, sysadmin - instance level). I would go back and check first the role that you created and make sure that it is not a member of any other roles that might cause an issue. db_datareader for instance shouldn't be a big deal. Second I would check public and make sure IT isn't a member of any other roles. Then I would check any other roles that the users might be a member of.

All that being said it's also possible you made a mistake when you set up your role and revoked instead of denyed (I've done that type of thing before).

Last but not least, you should change how you are doing this anyway. Remove all access from public. Create a role that has just the access you want them to have and add them to that role. It is generally considered a bad idea to grant access to public. Or for that matter to grant lots of access and then deny it to certain people. It's far better to accidently forget to grant someone access to something, than to forget to deny it.

I can guess you have the AutoClose option for database set to True. This is the default behavior when you create a database with Express Editions.

The mentioned error can occur exactly in this case. Actually the complete error message 615 states: "Could not find database ID %d, name '%.*ls'. The database may be offline. Wait a few minutes and try again." ... So it points that database could be closed during dropping.

So, go to DB properties, switch it to False and try again dropping it or use below script before dropping

ALTER DATABASE [MyDB] SET AUTO_CLOSE OFF 
GO

Many point out that is better to have AutoClose set to False. I found this article explaining a bit more about AutoClose: http://sqlmag.com/blog/worst-practice-allowing-autoclose-sql-server-databases

Small extension of the answer:

-- this works in standard SQL Server Editions, but NOT with Express Editions:
CREATE DATABASE [MyDB]
GO
DROP DATABASE [MyDB]
GO

-- this works in ALL SQL Server Editions
CREATE DATABASE [MyDB]
GO
ALTER DATABASE [MyDB] SET AUTO_CLOSE OFF 
GO
DROP DATABASE [MyDB]
GO