In a browser, visit a Lemmy instance's main page. The admins are listed in the sidebar. Note their usernames.

Still in a browser, go to your home instance and login. Hit the search magnifying glass in the top right corner, and do a user search for whichever admin you want to message. You may have to type in their whole ID in the form @exampleuser@instancename. When their name comes up in the search results, click on it and you should see a "Send Message" button as part of their profile.

That's me! Unfortunately, I don't have any clever solutions to active spamming other than responsive moderation, and better mod tools whenever they become available.

I was notifying instance admins of suspicious user totals that looked like swarms of auto-created bot accounts. Last time I checked, half a dozen admins that I contacted have deleted about 250,000 suspicious dormant accounts. It's not a solution to the problem, but it's something.