If the problem persists, you need to employ disciplinary action. This should start with the employee receiving a verbal warning for the first offense. All other policies about personal time use will apply.
Progressive disciplinary action will be applied, up to and including employment termination, if the dress code violations continue after the employee has been counseled.
Finally, if the employee's core job requires that he or she visit with customers and attend and interact at trade shows, employment termination may become your only option. You don't want to employ an individual who does not represent your company in the way you want your company represented. Your image must matter to you.