The vulnerability of steganography is that is has to pretend that signal is noise. Remove the noise - and the signal is gone. I'm pretty sure that the simplest gaussian blur will remove the spymark from any picture.
Or... add some noise. Just align the last bit of every pixel channel with a random bit sequence - and Bob's your uncle.
Or... add some noise. Just align the last bit of every pixel channel with a random bit sequence - and Bob's your uncle.