with a loud rippling crack.... lightning strikes venom and puts him down for the count (i'm sure the sentence finished something like that)
or thor lets go of one of his "godforce" blasts that staved off galactus. end of venom and the symbiote. thor could also let go of an anti matter blast (second to his godforce) end of venom....
thor could just pummel venom with his fist and hammer (he's a much better combatant).
thor wins pretty much no matter what scenario you put them in. unless you blindfold him, tie his hands and legs with adamantium changes and tranqualize him so he cant fight back. venom MIGHT win then.