Disallowing signatures for new members?
 

We'd like to forbid new members with fewer than 50 posts from having a signature.

Is there a mod or a built-in method to do this?

I'm assuming you have a usergroup for new members?

Admin CP -> Usergroups -> Usergroup Manager

Find signature options and disable for that usergroup

All our members are currently under one group, Registered Users

How can we set it so new members with < 50 posts automatically go into a "new members" group?

Usergroup -> Promotions -> Add new Promotion (set post count however you want)

The group they are currently in is the group for New Members. You will want to make a new group for Old Members and then promote users from the New Members to the Old Members group.

Ah okay, so I'll rename Registered Members to "new members" and create a promotion to move members with posts into "Registered Members"