By default campaign picklist should display blank value

Sales reps must attach a campaign to each opportunity they create. When they search for the campaign using magnifying glass, they see the first campaign DIC.....xxx. dozens of opportunities are tied to that campaign. The reps just pick the first one they see b/c there is no campaign. We would like to have None to be the first campaign to reduce amount of junk in our reporting and provide reps with a valid choice. thank you in advance.