What is the benefit of deleting a PR over just closing it? It seems like closing has the benefit of signaling what kinds of PRs aren't acceptable, which deleting would lose.
This is time consuming.
Unfortunately the PR and the PR author will forever be listed there and linking to their product anyway.
If they've been doing that to the other repo (and especially if they're just a spam account), there's a good chance using the "report" button and/or contacting GH support directly can yield positive results, up to the spam account being deleted (and the PR is usually deleted).
Unfortunately this doesn't scale.
Correct. I used to report them, but 3/4 years ago they made it more difficult to report anything because you have to explain what's wrong even when it's very clear.
This is particularly annoying because PRs also show up in the issue and in the issue list as "this issue has 3 PRs that will close it", when it's all. just. spam.