Non-toxic insect repellants.

Not even sure where to start looking for this.
My girlfriend (unwisely) left the garden in my hands, which means now that most of the flowers have dissapeared to be replaced by things of a tastier nature. In particular I've got chilli plants and plum tomatoes currently flowering (I'm in the southern hemisphere, so we've just rolled into spring).
There's a bit of spinach, a few garlic bulbs and some watermelon seeds that seem to have sprouted of their own accord.
(As a aside, how can you tell when garlic is ready to pull?)
I've spotted the odd snail around the place as well as small black flying things (they may be fruit flies, but I honestly don't know.).
I am looking for pointers as to what I should spread or spray around the place to reduce the chance of losing my "crop" to disease or insects. I don't want to hit everything with "Round-up", so I am looking for household tricks, "old wives tales" or something I can mixup without rubber gloves and a government permit.
(I drink a lot of tea, so if the leaves are good for anything (other than composting!) that could be handy.)
Thanks for any advice.
Matt
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