You have to deregister yourself as a resident in Belgium. This has to be done in 1 to 2 weeks prior your departure. Kindly address yourself to the Population Department of the municipality/city where you are settled.
It is essential to deregister yourself. If you don’t, a lot of problems could occur upon your next arrival in Belgium. Some possible problems could be: having troubles with your visa application or worse, a denial to entry Belgium.