Mobile - How to just use wifi and totally disable media net

There are several options to disable medianet, you have to do it specially if you don't have a data plan.

Delete the MediaNet connection
Start > Settings > Connections > Connections > Advanced > Select Networks > MEdia Net
Then click on:
Edit > Click and hold on the item next to radio button (MEdia Net).. when the popup screen (similar to windows right click) comes up click on delete. Then do the following. >> General Tab > Click the button "Delete Settings".
NOTE - You cannot delete settings for My ISP & Work Network. Which is fine.
If you ever need the settings back again.. run the Connection Setup Wizard again

Modaco Nodata
The MoDaCo No Data Utility allows you easily turn off your data connection so you don’t accidently use (especially while roaming!)
Get the application here

HD Tweak

These tools allow you to tweak your jewel very easily, to customize it and optimize it as you want.

It includes many tweaks with descriptions. Choose your tweak, check option, and simply apply it!

Get the application here