Hi Andre...
I'm assuming that you mean it adds this to the fig element? You can add the following to your rules file ..
element "fig"
{
attribute "frame" drop;
}
If you're using the default FM8 DITA apps, the rules file is at ..
FrameMaker8\Structure\xml\DITA\app\DITA-Topic-FM\topic.rules.txt
You'll also want to make the same change to the Book app rules file as well.
Cheers,
...scott