Flash Expsure

Situation:

Taking shots at a chamber of commerce expo with
lots of booths in High School gym with quite a bit of
light.

Gear:

EOS 350 D camera Speedlight 420 EX

Problem:

Camera insists on treating flash as fill flash, which
means if I am in shutter priority it locks the lens wide
open. Thus no depth of field.

What I want it to do:

Ignore ambient light, measure the pre flash, set some
reasonable apture and make the exposure.

How do I make it work that way ?