How do i turn internet access OFF on my samsung gt e2121b ???

ive got a cheap samsung mobile for texts and occasional phone calls, i will never use the internet connection on it, i want to disable it incase it is ever pressed by accident and uses up my credit on my pay and go

When i got it they sent me all the picture message and internet settings will be set up.

How do i disable the internet connection on the phone, can anyone advise??

Thanks in advance if you can help