Topically applied camomile lotion will soothe the itching and that should stop the main temptation for picking at bites, sores, ulcers and scabs. You can also buy proprietary anti-itching/irritation creams and ointments to do the same job.
Tea tree oil, as an oil or in soap, can fight infection to the same effect, as can witch hazel. Aloe Vera, as a topical gel or in soap, soothes and promotes healing, again helping to stop the need to pick at the source of the irritation.
If all that fails, you can apply something unpleasant to taste around (but not on top of) the wound. English mustard around the point will get on to the fingers and then almost invariably into the mouth - ouch! Aversion therapy.
If you are prone to insect bites, there are studies that suggest it is due to your pheromones - hidden smells your body naturally puts out. These can be short-circuited with various natural essences and oils or with artificial bug-repellent creams and sprays. Citronella, which has a lemony smell, can be bought as an oil and dabbed near to where you are most prone to bites, repelling many insects.