In your database, add a field that's called "approve" or whatever you want to call it. Set it to "enum" with "yes,no" as the available option.
In your script where the banner becomes viewable, add something like (this hasn't been tested, it's a basic snippet)
if($variable['approve']=="yes")
{
the rest of your script }
else
{ echo 'the message you want to appear if a banner is awaiting moderation'; }
You then need to make a second script that allows you to approve or unapprove the banner, which is a simple mysql update.