This spammer created multiple accounts on various Lemmy instances to post self advertisements. How do we combat this?

The answer to your question is good moderation and moderation tools.

Report the account.

Okay but are eyebrow transplants really a thing???
- @Risk But who is jenna?

We have a bigger problem and it's popularity. Some instances will want to become as big as possible, for resell value.
So they will be encouraged to let the maximum of people registering a new account, including bots and spammers. Because they make the numbers and they start the snowball effect.
Didn't you all looked a the most populated instances/magazine when you registered magz, and you didn't care about the mags with zero activity? Well, you are encouraging the process in a way.
People will be reluctant to defederate or to ban popular magazines from other subs. So the float of spammers is unlikely to stop, because the person who makes the decision, the admin of the instance, will welcome them. And you won't ban the instance or the mag.
We have to become more like a club.
- Some instances will want to become as big as possible, for resell value.
  What’s the resell value of an instance?
  
  About tree fiddy
  
  I think the person you replied to is still stuck in the mindset of a siloed product, like reddit. Federation across instances means that there's little commercial value in any specific instance, in my view. If any specific instance was sold, and an attempt to monetize arose, I suspect people would simply abandon it for a new instance.
  As an aside, lemmy/kbin need to implement a way to export/import follow lists.
  
  The value of an instance is a function of the number of his registered users.
  Facebook bought Instagram for the price of $30 per user.
  
  Whatever someone wants to pay for the kind of exposure this gives them.
- Your closing suggestion doesn't actually affect the process. Admins still run the joint. The admin just needs to care. Admins can even make it invite only if they wanted. Do you have any source for this marketplace of instances?
  
  So what will you do about an instance called "rofl.lol", which is huge, which has a lot of fans locally who want to keep interacting with it and which also allows spammers? Will you keep federating or will you defederate it?
  If you don't defederate this will become and endless whack-a-mole of bans with a drop of quality.
  Do you have any source for this marketplace of instances?
  I never talked about a marketplace of instances.

I don't know if this is quite the same thing, but @kersploosh was waging a war on bot accounts. Maybe they have some inside or can come up with something in conjunction with the admins.
Edit: that's @kersploosh @sh.itjust.works in case the one here is someone else.
- 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.

Hopefully several instance owners collaborate on a shared blocklist that will help combat this kind of spam and implement that back into their instances.
Lemmy at least does have a slur filter in place to block certain words that could be expanded to block spam like this. I don't know if kbin has something like that or not.

Ban and purge.
- How? I feel like it might be too easy for such accounts to be created. And they already pollute the fediverse before their accounts get banned. I think this will be more common until we have more proactive measures.
  
  Depending on implementation, the ban may also purge past posts so the "already pollute the fediverse" won't be that big of a problem.
  Can't get too proactive beyond suggesting admins implement captchas and the like. Email verification. And that won't stop all.
  I don't see this being much different than what occurred on reddit.
  
  Mutli-Accounting/Creating mass-accounts for spam-reasons has been super easy on all platforms so far. What do you reckon "more proactive measures" to be?

report the posts, the moderators will ban the profiles.
- Do kbin and lemmy allow you to report accounts to the instance admins? I’m still used to Mastodon, where you report spam to the instance admins and they boot the accounts. That wold solve the problem at the source rather than make magazine/community mods have to play whack-a-mole.
  
  Kbin has a report function, although I don't know if reports Federate. They might not.
  Lemmy does do reporting, although it's not clear whether it's just moderators, or whether the admins will also receive them.