Nope.
Have a look here:
- Watermarking [RawPedia]
- And this (old) pixls topic: How do you insert a signature using RawTherapee?
EDIT: I think I misinterpreted the question; This is not about adding a watermark or signature, but about removing them.
My answer basically stands though. Although you can indeed use RT’s local adjustments for this purpose, but it’s the hard way to do it. Use GIMP or Krita.