Is there a way to disable morphos' feature to automatically detect monitor specifications and adjusting of screenmodes?

I am using an HDMI splitter for simultaneous usage of my monitor and HDMI grabber (for streaming).
But as soon as i activate the splitter MorphOS will detect new screenmodes which results in an "out of range" on my monitor.
I'll need the screenmodes for my monitor permanently.

You can delete the ddc.library. Just remember to do this after updates.

Delete MOSSYS:Libs/ddc.library
Echo meh >MOSSYS:Libs/ddc.library
Protect MOSSYS:Libs/ddc.library -rwed

That should protect you from updates replacing the file.

Or, alternatively, completely break the MorphOS installer, depending on how smart it is...

Given that there's no provision for ddc.library in IWizard's update scripts, your advice isn't likely to change anything. The installer replaces the whole MorphOS directory and only a few select files are copied back from the old one.