When it comes to spot treating a pimple, ice is king. Simply putting an ice cube in a Ziploc®-type bag and applying it to a developing acne lesion twice a day for 5 minutes can dramatically increase the chances of preventing a full-grown zit. The trick is to catch it early. Once a pimple has passed a certain point, it is unlikely you can stop its progression.
Aside from ice, the next most important weapon in your spot-treatment arsenal is applying topical treatments. Applying benzoyl peroxide, waiting for it to dry, and then applying 10% glycolic acid is a tandem treatment, that when combined with icing, is very powerful.
All that is left after ice + topical treatment is tackling the developing pimple from the inside out with 30mg of zinc gluconate (found at any health food store) and an NSAID (like ibuprofen). While these oral anti-inflammatories are unlikely to prevent a pimple on their own, they are like the icing on the cake, and can be used in particularly dire circumstances, like if you notice a pimple developing and tomorrow is the prom or your wedding.