If scammers don't pay their fg back, should their IP just be banned?
They could just make another jsp account and not have that "repay" back on their shoulders.

No. Multi's do get found eventually either way. Plus, even if they dont pay back right away, some come back years later and repay. If they needed to be banned, they would have been banned. Plus, IP bans aren't all that effective either way.

What if they change their mind and want to repay?

Voted no

The repay process is the only possibility for the victims to receive back what they lost. I'm thinking about them. If we do what you are suggesting, this possibility will be removed. And you have more people who decide to change their mind, and to pay back than you can think.
Voted no.
This will never happen.

They could also change their ip, so what's your point?