Is there a way to identify alt accounts?

Short answer: no

Do you have to do IP tracking or something technical like that?

Long answer: yes

In this case they made it very clear who they were in another post when they were bitching about how nobody was grateful that they posted so much and were angry over being banned. "sanitation says hi" wasn't very subtle.

Always be aware of accounts made recently on instances with open signups, such as lemmy.today and sh.itjust.works. Some Lemmy apps will tell you when an account was recently made. Sometimes it's a legitimate new user, but more often than not it's a troll or ban evader.