[Venger]

Venomous, not poisonous :P

You're daddy long legs spiders (Pholcus phalangioides) will quite happily eat them for you, so will many other spiders, (these aren't native either and are making a much bigger threat to our native spiders (they will actively hunt our spiders where the Steatoda sp wont.)

Depending on which false widow you have (it applies to the Steatoda family, not to one specific spider), some are native, some aren't.
But if you're talking about S.nobilis they've been here for over 200 years (ish) before you start waiving the 'immigrant' flag around, they're only spreading due to the warming of the climate.

They produce so many young because they become dinner for many things. Birds, amphibians, mammals, and other invertebrates will prey on them (on all spiderlings really, rather than this one species), only a few will make it to adulthood.

Even where their more venomous relatives live (said black widows) no one has died from their bites - bites are still rare (while obviously the bites may be severe and unpleasant especially if you have an allergic reaction to them)

You'll just have to adapt your lifestyle I'm afraid - insecticides will kill ALL spiders rather than one species.