How can I disable mobile data connectivity on my Sanyo Zio? I have a pay as you go Android phone. I only pay 10 dollars per 3 months (or something like that). That means that every cent counts before I run out of money.

I want this phone for emergency reasons only and for internet so I don't use it to call. Yet my money on the account still runs out. When I check my statement I see a lot of charges for few to as much as 30 cents. I use my internet when I'm connected to wifi only but I think I'm still getting charged for connecting through my 3g network.

Is there any way to disable the connection with the data plan so the only internet that will work is wifi? This way I will not get charged. 

I have tried changing the apn under settings and with apps. My phone seems incapable of even doing that. I wonder if it's some scheme to get me to pay for data without me noticing. I would still prefer a solution that allows me to call and receive calls at all times since I might receive emergency calls or need to make them. 
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Update
I downloaded an app called easyswitch and it allowed me to disable the 3g network the problem is that it's a trial that lasts a week. if anyone knows a free app that will do it please let me know. I also rooted my phone but don't really want to install a whole new rom on it.

After using easy switch for a while i realized that it keeps allowing 3g sometimes (sometimes it doesnt) I dont know what its depended on maybe if my phone restarts the easy switch settings restart as well? I think i will just install a new rom.
"Data Roaming" just refers to using data when connected to a foreign network (i.e. not your providers "home network").

There should be another option available for "mobile network" or "mobile data" (the phrasing may differ between devices), as the following screenshot shows:



Look out for the red frame at the end. If you find something similar, remove the checkmark. If not, simply remove it everywhere where it sound similar. Remember which ones you removed, then enable them again one by one until you find the "evil doer".