How can the rate of reaction be decreased?
There are a few ways to decrease the rate of reaction which I shall mention.
Perhaps the most obvious answer is to decrease the temperature of the reaction: this slows down the particles and, thus, reduces the likelihood of successful collisions between the reactant particles.
You could also use a larger lump of solid in your reaction (if your reaction includes a solid) - this decreases the overall surface area exposure. If you were to cut the solid into smaller pieces, more surface area would be exposed and, therefore, the rate would increase.
Small chunks -> Larger surface area
Large chunks -> Smaller surface area
You can also decrease the concentration of any liquids in your reaction - Two mole concentration will react faster than a 0.6 mole concentration, for example.