How can i stop my PDA Mobile connecting to the internet while i'm abroad?

It is an O2 Orbit II XDA, while i'm at home i don't mind it connecting because i've got unlimited web browsing bolt-on. while i'm abroad it cost quite a lot so i want to temporarily disable it. Do you have any advise on how to make my mobile just work as just a mobile for a while(make & receive texts and calls)?