Renae, already your landscaping company is on the defensive, which is never a good sign for how the relationship is going. The first thing you can do is go through the contract and see if that specific item is part of your scope of work. Once you know that, you approach the situation accordingly. If it is, then you simply call the contractor and ask to meet with him. When he gets there, you can be nice (usually the most effective) or you can be direct (usually promotes more defensiveness, not the most pleasant.) Show him the scope of work and tell him that you personally are experiencing this issue. He should immediately respond by telling you it won't happen again and apologize.
The second way, if this is not covered in your scope of work, is to meet with the foreman(person) while he/she is there during your service and point it out to him/her. You don't necessarily have to be the one to do this. A ot of times your Maintenance Supervisor handles this. They should comply from that point on, especially since your on site team will be watching. Here's how I have done this: I grab some bottled water (cold) and start distributing it to the guy walking around with the string trimmer, etc. and when I get to the guy with the blower, I take him aside, hand him his water, and ask him to take a break with me. While I'm there shooting the breeze, I casually say, "Hey, I notice you're blowing the clippings onto the sidewalk and I notice some of it is getting onto the cars. Could you please blow the clippings back onto the grass?"
Then follow up one or both conversations with a letter to the contractor simply stating that you chatted with the on site crew and asked them to keep the clippings off the sidewalk and cars and how much you would appreciate that. It will cut down on the complaints from your Residents and staff regarding the issue.
When you negotiate your next lawn care contract, add the clause in there about all clippings must be blown away from the parking lot. Then if they do it and don't stop, you can withhold payment until they comply (which is of course the least nice way to have to handle this situation.)
Whatever you do, remain calm and professional and nice about it. Your actions are a reflection of your management company and yourself as a manager. Show the landscaper and all contractors respect and you won't have any problems.