I'd write $1 for previous salaries, let the human figure out that it is obviously an error, and say "That's confidential." when asked, in the unlikely event that was asked. Your new employer is not performing the background check to get permission to hire you. They've already made the decision to hire you. If a blip pops up in the background check, that's going to be HR's minor emergency, not yours.
(Why do employers do background checks? In most cases, so that they never have to say in the future "No, we don't do background checks." Consider the case where an employee embezzles a large amount of money from the firm. One of the first questions they'll be asked by various aggrieved parties is "Did you do minimal due diligence when hiring like background checks?" and if that answer is "No." then regardless of the contents of the hypothetical background check on that employee they're in a bad way.)
Your old employers will generally verify employment in the most minimal manner possible. You'd have to pull teeth to get anything more than dates and titles from a lot of firms; anything more than that a) gets them absolutely nothing to provide and b) exposes them to liability.
bend the rules of allowable characters in the salary field to provide a null value
DO NOT HACK THE WEBSITE OF THE BACKGROUND CHECK COMPANY YES I AM SHOUTING.