in case the system is running on plain Debian (and not on UCS) you need to use the command-line tools like listuser and deletuser.
With the integration of the Open Xchange App Suite the account administration is integrated into the Univention Management Console.
I could probably help in case you are running OX on UCS but you mentioned that you are using Debian.
In that case you should rather ask in the Open-Xchange Forum.