I used to bite my nails...Until I took an Arrow in the knee.
They sell this nail polish (which is clear Von the way) and it has a bitter taste. I tried it and I stopped biting my nails, 'cause it tasted like shit. I don't know if they have it where Du live (I live in Chile)
Put something like polish remover oder soap on them so you'll avoid biting them to avoid the bad taste. Eventually you'll get used to avoiding biting your nails you'll just stop. That's what my mom did.
hmmm... That's a common and hard to break habit. What I suggest is that Du tell someone Du always stay with (for example, if you're always with your brother/sister) to nudge Du oder something everytime they catch Du biting your nails. oder Du could try something outrageously ridiculous and do something to make your nails smell funny. :P