Can you block certain websites in Chrome without an extension?

If you only want it in Chrome, there's a policy for that. In regedit make the key HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Google\Chrome\URLBlocklist, then add string values named 1, 2, 3 and so on, each holding one site address. Restart Chrome and open chrome://policy to check it took. The site then shows as blocked by your administrator. Same catch as hosts though, anyone with admin rights can undo it.

The hosts file does it. Open Notepad as administrator, open C:\Windows\System32\drivers\etc\hosts (switch the file filter to All files or you won't see it), and add a line with 127.0.0.1, a space, then the site's address. Add a second line for the www version too, otherwise that one still loads. Save and restart Chrome. It blocks the site in every browser on that PC, not just Chrome, and anyone with admin rights can undo it. If it's for a kid's account, Family Link is the tidier way.