Manually select / deselect phone provider

I am living close to the border Germany France and since the Iphone 4s I have a problem with the iphone that neither the Apple support nor the change of the phone (to the Iphone 5) could solve:

I am manually switched to my german provider "Vodafone.de"

Without changing anything the phone sometimes connects to a french provider when close to the french border, in this case of the screenshot "SFR"

How can I prevent the phone of selecting a certain provider? I thought this is done by manuall selecting Vodafone.de but obviously this is not the case.