If you're not sure that disabling the data roaming is certain enough, set you mail app to check for mail manually and then do a full power down and restart. Once back up, don't open any web apps (including the minor ones like Weather and Stocks), and you should be fine. If you like, you can reset your data usage counter just to make sure that nothing is getting through.
FWIW, AT&T does offer an international data roaming package that gives you 20MB for $25/month. 20MB is not much, but (depending on how often you roam) it's plenty for mail, maps (sans satellite imagery) web surfing and using the weather etc. apps. If you get crazy with YouTube, you're in trouble. If you take this package up, resetting your usage counter is very helpful to keep you within your 20 megs. The catch is that it requires a minimum of 12 months subscription, so you can't buy it just for this one trip.