Perhaps this should be in herbalism or misc?
Eat plenty of garlic, if you can. Garlic cleans the blood and actually gives it a weird taste, which mosquitoes don't like. Other than that, ginger is also good to have in a little satin bag, perhaps.
Citrus is also a good way to keep them away, especially lemon. They dislike it very much as it is too overpowering.
Other than that, there's no spell that can wipe out the entire insect population. Don't you think that is a bit harsh? I agree with one of the comments above. They are here for a purpose. Creatures like the butterfly are so beautiful.
If you don't want to use either of those, simply buy insect repellent and they won't bother you.