Answered in another thread but I'll post it here as well incase someone comes across this one.
Goto your postbit template
Find
Code:
<if condition="$post['signature']">
Change it to
Code:
<if condition="$post['signature'] AND THIS_SCRIPT != 'private'">
You'll also need to do that in your Postbit_legacy template if you use it.
This will disable the sig in private messages.