AAAnimalControl.com and GardeningKnowHow.com suggest the only sure way to get rid of opossums is to set traps to capture them, and then have an authorized wildlife expert relocate them. Relocating opossums is illegal in some states without proper authorization. Killing opossums is also illegal in many areas.
Repellents tend to offer little relief when opossums take residence under a porch or in a back yard. However, you can make the area less appealing to opossums that are looking for a new home by keeping your yard clean, trimming any brush or shrubbery that opossums could use as a hiding place, and even put fencing up around the yard to make it more difficult to enter. However, AAAnimalControl.com states that preventative measures are rarely very effective against opossums.