Can this be done in the CP, or if a hack is needed is it just a simple change to deactivate their capability to erase users (incl. their posts)?

Well it is not in stock code.
So it would be a mod.
I am not aware of one that exists at the moment.

Possibly ask at UBBDev.com.

I have access to edit users and block them from the cp here as a mod; so i'm pretty sure it's in the stock code... I do not have the ability to delete them.

Yep, you are correct Giz.
I guess I read the question to quickly.
Mods can not actually delete a member but they can do other items such as ban them or delete or move posts.

Well it depends on permissions. I set myself as global mod, took away permissions to delete a member and was able to do just what was asked from stock code. It takes a bit of doing and understanding the permissions but can be done.


Dunny

The only setting you can change is the "Can edit users" one. So either they can ban and do other stuff, or they can't touch users at all.

So, back to the original question.
They really should not be able to delete a member. Unless you placed them in the admin group also.

I donno, what foes the "full cp access" perm do in this regard?

I tried that already in 7.x.
It does not allow more permissions in member management.

Odd, i think that's a bug... I mean, if a mod has access to destroy my forum, I don't think there is much damage in allowing them to delete users lol...