Furthermore (not an apt specialist and N900-retired):
this apt preference is related to label (-l) Extras and does not restrict Community repositories.
And also you would downgrade some packages installed from Extras-testing / -devel.

So maybe just remove (hash out) community-devel from repos.list?

There may be possibilities to use origin or archive option from apt but why (above is simpler)?
https://wiki.ubuntuusers.de/Apt-Pinning/