Cubase automatically creates one (and only one 
) Default Control Room preset, which stores the settings of the last session. These settings are re-applied the next time the application is started.
The enormous number of Default presets in your list is unusual. You can delete them manually in Audio Connections - Control Room. Or, if you are familiar with editing XML files, you can edit the ControlRoomPresets.pxml file accordingly. Preset objects begin with
<obj class="FPreset" ID="[number]">
and end with
</obj>
If Default presets continue to be added, you can try to write-protect the ControlRoomPresets.pxml file.