It's your choice which folders you choose to share! You can change the default setting in Menu-Options. If necessary, set up a special share folder & allow people only to share from this folder using the above technique. I hope I understood you correctly. If not, then get back to us.
If it's folders on 'your' network that you want to protect from others on 'the same network', then there are techniques. I'm not a pc person so I'd leave that up to others. For Mac there's a program called Ghost which makes a folder invisible & even passwd protected. I'm sure there's plenty of apps that do something comparable for pc.