GSM library and roaming

Hi,
I am trying to make device that will read some sensors and send them via sms.
I use this shield

http://www.ebay.com/itm/SIM900-GPRS-GSM-Shield-Development-Board-Module-Kit-For-Arduino-Compatible-New-/291411734213?hash=item43d97e2ec5:g:HvMAAOSwqu9VC6Jw

along with arduino mega and all is working just fine, and project is almost done.
But sensoring device is near state border, and from time to time my shield goes to another network from country next to mine(roaming). That happens mostly when there is some storm and signal from roaming is stronger than my home gsm network. That is something that i must fix before making it operational since roaming sms price is not included in my sensoring gsm plan. So what i would like to have is fixate to my home network without roaming possibility even when there is no home network signal.
I use arduino gsm library.
Is there any way to accomplish this?
Maybe with some AT commands like:
GSM3ShieldV1DirectModemProvider modemAccess;
modemAccess.writeModemCommand(AT_COMMAND,tDelay);

I tried but couldnt figured it out.
Thanks in advance

drdevice:
So what i would like to have is fixate to my home network without roaming possibility even when there is no home network signal.

Is this something you can work out with your service provider?

Well it is possible to work that out with my service provider at the moment, but it wont be always possible so i am trying to figure out how to do that on my own.