I highly doubt that.
BW can't punish an account when all it has done is allow another player into their stronghold.
What if a spammer visits a publicly listed stronghold and uses it's mailbox?
What if the spammer joins a guild and uses their ship's mailbox? Do you really think that a credit seller is going to be deterred just because they can't find a mailbox on a starter planet?
Your suggestion is terribly flawed and any credit seller would laugh at you for thinking that this would stop the spam.
The people you're trying to stop are more clever, have much more free time and are much more motivated than you realize.