There are always a couple of bars of rubbish tagged along a tune. So, for example if I have a 12 bar blues in Bb, it will add on 2 bars of F7 at the end, where the whole thing just dies down in untidy fashion - sounds a like a band that doesn't have it together. How do I get it to stop at the end of the 12th bar ?

I should have known this by now, but since I've always used it as a jam tool ...