U can do it but need to use a proxy server, the default unifi router dun have this feature others i dunno. another solution is using content filter but need to install on the device. can use such as this one (as an example..i'm not responsible if it cause problems ).

http://www1.k9webprotection.com/

this type of content filtering have some annoyance also.. such as blocking some legitimate website (miss-categorization..etc). maybe u can try which solution works for u.. good luck.