how to turn voice control off iPhone 4

Hello, my girlfriend has the iphone 4 and she has dropped it multiple times and now the home button is sorta stuck. Voice control comes up constantly and calls people while in her pocket and she hates it! I was wanting to know if their was a way to turn this feature off, her warranty is voided because we switched to my parents plan so cant send it in =[ someone please help this is very frustraiting for both of us! thank you for taking time and reading this