The only plant that I have heard of doing this is the annual plant Tagetes minuta. It is sold by Chiltern Seeds (google it). Bit late now to sow though. It's species name comes from the flower size, not the plants size which can be up to 6ft in height! The flowers have a look more like Groundsel. The roots are supposed to secrete a chemical that kills it.

Thank you for responding fractal, that is the one i found online its not the most atractive though i was wondering if there were any others. someone once told me geraniums were good also, i was wondering if anyone else had heard this or if i needed a was a particular type.