Talking personal experience you CANNOT stop an asshole. Senna was famous for it and a great racer too. Back to the OP the best ways are to be patient, tune your car to the hilt and if you have to, get nasty. starting a race, try and spot the gaps and use the turbo start always. Contrary to some philosophy, this is the safest way to start. You do have an increased risk of being baulked if you don't think ahead, but since everyone heads for the inside of a turn, you head outside and use the space. most times you will be up the field and in clearer air, possibly misplaced (unless the next turn is in your favour). This is not often a problem as a lot of racers take each other out on the first corners and coming from the outside of a turn gives you momentum and the chance to get on line without too much grief after everyone has gone off (just give space to those who haven't wiped out).
Next when you have the racing line everything is in the braking. Use it sparingly but use it often. Light braking at the right moment with a bit of trail braking can set up a legit block pass. You may still get over taken, but with the field now thinning you have a fighting chance
Now for the pitting. Stay in pitting position (door handle to door handle. The less room you give them, the harder for them to force you into a spin). Don't get ahead of the competition too much until you have the racing line and can move cleanly in front. If they are going to ram make sure they catch you square. If you are likely to spin, give them the place buy turning out and getting off the gas (and sneak a tap later). If you are approaching a turn, brake early and lightly and be on the inside of the turn if at all possible. If you know you are going to get hit, brake harder. The slower you go the less likely you are to spin and if appropriate, pit them after as they will be trying to get away, but will have less momentum than you and won't figure you to spin them. with them gone, get on with racing.
All you need to win in a lead, and an inch is as good as a mile. If you are squarely side on, they can only push, not spin and that's important. If you are side on and a nose a head, then you win. If you car is maxed then your nose will be ahead. Racing is about positioning, driving and if need be rubbing
Some races you will win, some not, but as a tip stick with the same crowd. You'll learn the liabilities who will crash at the start and who the hit men are. Quite often you will get strikers who are there to let others win. Be careful and move on. These aren't races you're going to win,ever.
Hope this helps