If I do want to relax and not be bothered by them I have always found that a No thank, or for the very persistent sellers just a very firm NO works
Of course this does work, however, when you have six turn up in as many minutes you start to become tired of saying "no thank you", "I will look towards the end of my holiday", "I don't want anything today, thank you" etc, etc. They all talk to each other so they know what you have said to the others, but rather than respect your privacy they continue to pester. Just as they have the right to earn a living, I have the right to privacy.
A few years ago we became friendly with a beach seller and it didn't take him long to start asking us for things (mobile phones, crocs and donations towards his childrens' education etc.), so we decided we weren't going to become involved ever again.
We have spoken to other tourists who fell for the same lines and later regretted trying to help when for example, those they helped moaned about the mobile phone they were given or found out the phone was sold on.