Is the buckle a seperate material group ? if it is then set diffuse and specular color to black and transparency to 1 .

If its not then you can try the grouping tool , select the entire hat, deselect the buckle, and use the spawn prop option to make a new version of the hat.

Or you could select the buckle and make it a new material group, then set it to transparent.

Next method is a transparency map , or load it into a modelling program o edit it.

A lot will depend on if the hat is complete behind the buckle.