How do you report someone? (spoofer with 4 accounts)

There's one in my neighborhood. He has over 100 accounts. His accounts are nameXXX his username is then followed by a 3 digit number. I've seen dozens of them.

I work in a wildlife park. I can see when/if anyone is near one of the gyms. Even when nobody is in eyesight for hundreds of yards this person battles a gym, then heads a mile away and battles another one right after.

I don't care. It is what it is. It's never going to be stopped. This person has had free reign over the park for over a year. All his accounts are between 38 and 40.

You just have to live with it. With millions of players it;s completely unrealistic to expect niantic to chase down every single complaint. Instead they do broad sweeping nets and attempt to catch them that way.

https://niantic.helpshift.com/a/pokemon-go/?p=web

Press the "Contact Us" button and fill the form. Reasons for the report include everything from bugs to reporting other players to any other reason to report to Niantic.

Unfortunately it's very difficult to prove multiaccounting. If they spoof, Niantic may be able to verify that from their own data, which apparently can lead to a ban.

But note that a lot of people in my community including me have been reporting the same spoofer for well over a year, and he’s still here.

There's really no way to prove that this person is spoofing. Niantic has to catch them using 3rd party software that they deem malicious.
My little brother used to spoof to San Francisco, and it bothered me that he would do it, but Niantic ended up catching it and suspending his account for a day. They have to get caught so many times in order to actually be banned.
Correct me if I'm wrong but I believe you first receive a soft ban, then a day suspension, then a week or month, then perma ban.
Like someone said above, they try to catch a bunch of people using the same software all at once. It's kind of hard to catch these players spoofing because they all know how to hide it.
You could have a bunch of people from your community to try to contact Niantic, but it still might not be enough.
There is a difference between spoofing and having multiple accounts. Spoofing is easier to catch, while having multiple accounts is near impossible to catch. Most people have multiple phones just for PoGo. While Niantic says it's against the rules, they do nothing to enforce it.
At this point, it sucks to hear, but as others were saying you'll just have to deal with it.

There's a specific gym around me that spoofers just love. I have no idea why it's just the one that gets fought over so, so much, but it's multiple times each day. I've developed an easy system for when I expect a spoofer to attack or retaliate.

I take a screenshot after putting my pokemon into a gym, then walk around the area playing the game as usual. If a spoofer attacks, I wait until I'm kicked out of the gym, then take a screenshot of the spoofer and his/her mon. Finally, I take a few pictures of the area.

Because all of the pictures on smartphones have built-in time codes, it's easy to develop a chain of evidence that shows A) you're physically at the gym, B) no other players are, and C) you were still attacked by xXxDongSlayer420xXx. I've seen this system lead to spoofers disappearing from my area, and others have gotten smarter about how they play the game.

It works for me, but I mostly play at night, and I live in a suburb. Your own mileage may vary, but it's a start. When taking pictures of a gym's real world area, try to cover as much ground as you can in as few images as you can. I keep it to two or three pics along with the two screenshots. Also, try to be courteous to other people when you're snapping photos.