Is there a way to review only characters (no words) from a lesson ?

You can do this manually by creating a list which only contains the characters you want to study and then only studying that list. There is no automatic way to do this.

Are you trying to cram new characters, or is it taking too long to review the words?

You can export the relevant words from the skritter list, then analyse on http://hskhsk.pythonanywhere.com/hanzi to extract all the unique characters, then reimport as a new list in skritter, then just study that list. Shouldn't be too bad, just a bit of cut and pasting.