How important is it for Sales guys to come into the office on time, everyday! As HR professionals, we need to look at things from the business point of view and flex our rules to suit what's best for the company and for the employees. Good sales guys means generating revenue for the company, meeting targets and having the company function smoothly. These guys are perhaps visiting the market late in the evening and even working out of home since where sales is concerned, there aint no timing at all!
Can you thus put them on to a 'flexi' time! Let them know what needs to be done from the office, and as long that that is being done seamlessly, need not worry about their timing.
if push comes to shove and the company cannot be flexible with them, start the day with something interesting for a few weeks....something that entices them to be there on time, rather than punishing them (since it did not work anyways)!