How to block unwanted people in Whatsapp?

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I am currently using Whatsapp with my Samsung Android phone. I am very new to this application, but I find it really useful and convenience when chatting with friends. It can totally replace sms as in theory it is free to use with free wifi network. However, some of my old friends also use whatsapp as it is very prevalent nowadays, and I don't want them to see me online or send messages to bother me, and more importantly, I don't want to change my mobile phone number just because of these !@#$%^&* people, how can I block them in whatsapp??