Don't bother to try. Aerial roots are an important part of the plant's respiration. It's fine the way it is.
OK, I shall give up and let it be. I am so desperate for some new roots inside the medium but there is a new one branching from an old root and growing pretty fast ... hopefully the old roots will not die due to the transition from bark to S/H. The only new one growing from the crown want to be aerial.