Double click on the Home key. When the multitask bar shows up tap-hold one of the icons until they start to wiggle. Then tap on the red minus sign on the AIM app.
This closes any app frozen in RAM or running in the background. Note, not all the apps listed are actually in RAM or running. It depends. Most of the time only the last 4 or 5 are in RAM, and only those that have specific types of background tasks are running.
iOS closes apps as needed to free up RAM for new tasks.