You would use a media query the same way, except this time you write the query to hide the object if the screen width is wider than a cell phone. If you are using a text widget to display your mobile ad, then the text widget is going to have a unique ID associated with it that looks like
text-##, where
## is some number. You can see what this number is by viewing the source of your page or using a web debugging tool. For example, if one of your mobile text widgets has an ID of 45, then you would add this CSS:
Code:
/* Hide these ads when screen width goes above 420px */
@media only screen and (min-width: 420px) {
#text-45 {
display: none;
}
}