andrewjs18 if you are writing your own migration script, you will have to set the suspended_until value on the users table. I believe indefinite suspensions are represented as a value in the year 2037.
In Flarum I believe suspended users can still log in, and change some preferences, but they cannot perform any action. It's the same state an account that was just created but not yet activated would be, except you cannot re-activate it yourself.
What you could also do if those users don't have any posts, or if you don't care about the link between those posts and users to remain, is to not migrate the users at all. Don't create them in the Flarum database. And if you don't want those users to return, you could save their email addresses in one of the email blacklist extensions.
