You must say no to the things that distract you away from your important work. You must say no to interruptions and infringements on your time.
You need to set boundaries for employees, vendors, media reps, and even charities. All of these groups are important to be sure. Tell your media reps and vendor reps that they must have appointments to see you from now on.
The days of them walking in on you are over. Be polite but firm. You will need to send a few out of your door because they forget your new policy. Be sure that you have spoken to them or emailed them about your new policy before you start enforcing it. You will only need to say no once and they will conform to your new policy as well. You are also doing them a favor by forcing them to be more organized.