How can I block ads from using my cel phone data plan?
You can turn off access to mobile data for all apps on the Samsung Galaxy 
Light by going to Settings->Connections->More Networks->Mobile networks and clearing the checkbox next to Mobile data This won't allow any data over the cell network, but data over wifi is still active. Note that this will likely have a negative impact on usability of a number of apps that require connectivity to operate as expected if you aren't connected via wifi, so you wmay want to flip it back on if your heading out.