I have a prepaid sim card with 1-2 call and a Samsung A7 phone. Sometimes the mobile data function switches itself on and the money is gone in minutes. How do I prevent this from happening?

Change APN entry from Automatic to Manual and make sure there are no APN details entered. Mobile data turns itself on after you have turned it off? Really?

You could also install a firewall which would control what apps are allowed internet access.

I finally had DTAC block the use of my "voice" minutes for Data, since like the OP if it get turned on, your pre paid minutes go pretty quickly, since many apps use it just to log on even if you don't use the app

Don't know about the OP's phone but with my Nexus 5, Marshmallow (6.0): Settings > Data Usage > Cellular data > Off

I finally had DTAC block the use of my "voice" minutes for Data, since like the OP if it get turned on, your pre paid minutes go pretty quickly, since many apps use it just to log on even if you don't use the app

Don't know about the OP's phone but with my Nexus 5, Marshmallow (6.0): Settings > Data Usage > Cellular data > Off

Thanks for the tip. I finally found a similar option in my Galaxy E7 and will see how that works.

Here is how it's done on a Samsung (and other?) device if anyone has the same problem:

http://www.technipages.com/android-enable-disable-background-data