Use bug bomb(s) first to kill the existing spiders. Then apply permethrin diluted with water to a 0.5% solution, spraying at all entry paths to the garage, edges, and dark areas.
The idea is to not just prevent spiders from returning. It's also to deprive them of a reason to return by removing their food supply. Permethrin is a wide-spectrum insecticide and repellant. It has very low toxicity to humans, most mammals, and birds. Not good around cats or fish. It has good persistence (6-8 weeks).
Install a bike alarm inside the seat tube. They're hard to defeat without tools. Mine alternately flashes bright blue and red light strips epoxied to the frame. In California, that's a guaranteed police stop (blue lights are only allowed on law enforcement vehicles), plus it makes everyone look at the thief. There's also a relay that shorts out the motor, making it a bitch to pedal. If that and the 120 dB alarm doesn't stop them, 30 seconds later two metal strips under the seat fabric get powered from a 55 kV mosquito zapper circuit.