While I personally have only been subject to a single couple day chat ban, I have a couple guild members that were perma chat banned.
Sadly this also includes guild chat and whispers, leaving players with no way to communicate except mail which is really detrimental to the gameplay.
Whether the perma chat ban was justified or not, this is paramount to just losing that player as well.
My suggestion would be as followed: Have chat bans include all GLOBAL channels only. If guilds are displeased with their members they can always just choose to kick that member.