this sounds like the work of slugs/snails. yes cut the stem down, as all the buds are gone they wont flower now.

I tried growing these pretty plants once but they were a magnet for slugs and snails which ate them away completely. They never recovered. You need to take preventative steps when they start growing again.

Yes, It was snails. I looked last night and there they were chomping away! Don't they know how much plants cost? Do they even care?
Well, sorry but the pellets have gone down now. I know we should not kill wild life but apart from crushed egg shells what else can we do for a quick result?
Thx for your replies.

You can try a thick layer of grit all around the plants once they've died down but you'll probably need to do it again next year. Having said that It's been my experience that slugs and snails climb up nearby plants and jump across!

Copper rings are good but only if the leaves aren't close to other plants. My perennial lobelia will be in a pot next year so that copper tape around the pot will stop those creatures. Again though, need to keep away from other plants or as Arbuthnot says, they will jump across or dangle down and fall on the juicy leaves of lobelis. I have never had any luck with eggshells, grit or porridge oats. I think i read somewhere that garlic spray has some effect.

Well, I never knew snails could jump! Thanks again.

Maybe snails can't jump but that's the only reason I can come up with for them still eating protected plants.

I never found copper wire or tape to work. A good layer of grit in pots works a treat for me but there's nothing wrong with a belt and braces approach.