HELP CLOSING MAIL AND GOOGLE MAPS iPhone 3G

Dear guys,
My 3gs suffer from battery drain caused by background 3g traffic data activity.
Even if I have no background running app, my phone keeps sending receiving huge mb of data.
Ive downloaded from appstore an app, system lite, which has revealed that, between runnin apps, there are mail and google maps. Maybe this colud cause traffic data.
How can I close them? Obviously, double pressing home button doesnt show these apps running....
Thanks in advance.
Daniele
Iphone 3gs iOS 4