How to sever the link to old repo that we forked?

Our team is working on an open source mod for a game called Cities: Skylines; we are having some troubles with GitHub continually choosing the wrong repo for PRs.

Prviously the mod was maintained by a single developer who has since left the community, his repo is:

Some time ago, we forked that repo to:

And more recently we set up a CitiesSkylinesMods git org and moved the repo from krzychu124 to it’s current home at the git org:

The problem we are having is that when someone wants to create a PR, GitHub always assumes it should be in the VictorPhilipp repo.

Here’s some screenshots of what is happening:

244 PRs later, having to do those steps every time… it’s just tiring and irritating. Worse, it’s confusing and cumbersome for new or ad-hoc contributors , who keep getting their PRs sent to the wrong repo.

We have tried reaching out to VictorPhilipp to see if he would archive the old repo, but to no avail.

Is there some way we can convince GitHub that if someone creates a branch in CitiesSkylinesMods/TMPE then that is most likely where they want their PR to be created, not some abandonware repo we forked from over a year ago? Ideally we would like to retain the link to the repo we forked, to attribute all the work the original developer did. But we don’t want PRs going in to that older repo because it’s dead and no longer maintained.

Hi @aubergine10,

Thanks for being here. It is possible to detach this repository from its current network so the base is the current repo. Our private support team can help with that. 

@aubergine10, your question is best answered privately.
I am going to create ticket on your behalf and lock this thread.
We will follow up with you in the ticket.