How To Block People From mac Wifi At Home

Hello !

You can create admin account and don't share password .

Suggested apple article : OS X El Capitan: administrator

Your question isn't specific enough. Block who? People using same Mac? Other people living in your home who use the same Wifi connection? Or people (like neighbors) who tap into your Wifi signal?

Create a strong password.

iMac: Use ethernet & shut off Wi-Fi.