I don't believe the Hub 3 has the feature to block certain website or words I'm afraid. I think the only way to have this facility is to manage the individual device with some sort of software or to use a 3rd party router.
Really sorry about this,
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the hub 1 had that option., the improved hub 2 and the even more improved hub 3 don't have it.
you can do it at device level by changing to open dns and using a free account with them to set custom blocks. that is what I do to block annoying click bait from taboola etc in new stories. you can also choose to block any from about 30 preset categories