How to stop lan users viewing unnecessary websites

Hi everyone,
I have TPLink Router TL-WR740N, through which my colleagues & myself are surfing. I do not have any client server setup, all systems are independent. I still want to stop my colleagues to view unofficial websites I.e. youtube, facebook, etc., during the working hours. How to configure my router to do so? I donot have to limit their usage, as they have GBs of official downloads, uploads. Just to stop unnecessary websites. Please guide me. Thanks for reading.
 
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