No! The Western Ribbon Snake is not poisonous AT ALL. They do not constrict and they do not carry ANY venom (not even a little). They live on a diet of small prey (tad poles, slugs, small frogs, small fish, etc.). They have a very strong jaw which they use to over-power thier prey. That is pretty much thier only weapon (other than speed). They are considered a gardener's friend and will probably just go hide in some shady brush if you come near. When handled they do sometimes bite but it doesn't really hurt. If you have to pick one up and would rather not be bitten, just grab it behind the head.
No, well not to humans at least, to small insects yes but they aren't anything to be afraid of