If the frame is steel then you could get it welded up, should be cheap and quickk to do, but you'd lose the paint around that area, as it's plain red it'd be fairly easy to patch it up though to a reasonable standard.
If it's aluminium then errr..... as above drill a hole at the end of the crack and hopefully that'll stop it in its tracks.
How did you not notice that you were cutting through the seat tube as well as the seat post?!