If this is actually how the bonus advertisement was shown on the deposit page, then they have no choice but to Honor it, it is completely unprofessional and a scam if they don't.

If this bonus was for first time depositors, then they should have completely stated that in the displayed ad like - for first time depositors only, or better still, completely remove the ad for those or users that are not depositing for the first time.

They will have to honor the bonus since they didn't state whether it was for first time depositors or not.