What can be done to stop this? Should anything be done to stop this practice?

Congressmen have the ability to pass laws and hire private companies for tasks need by the government. For example, congress passes a law saying company "X" will be paid to produce all the bullets for the American military. What congressmen do shortly before such laws are passed is buy stock in the company they're about to hire, and thus they make huge profits investing in that company. Should anything be done to stop this practice? What can be done to stop this?