My Winston was the worst for ignoring "no" i tried the stern no, but he just looked and wagged his tail too - then carried on. I have to bark it out and make a big clap. The rest of the family thru the house jump and stop whatever theyre doing as well!
but thankfully I rarely have to say no, its saved for chewing anything but his toys, nipping at any fabric other than his rucky and toilet anywhere but his place. It broke my heart to do it but he gets cuddles and love for the other 23 hours 58 minutes of the day! it wouldnt be fair to him if he grew up without manners cos all it would take would be nipping a child and tragedy for him! I found that screaming out in pain and then wimpering if he nips you or bites during playing leads to licks and kisses as he thinks hes hurt you. lol, he's only 9 weeks and he's only been here 4 days but I've been firm and he doesnt do those things now. He's a super fast learner, who also knows fetch now - and does very very well!!!
But he too hoovers up everything infront of his eyes, he's so young he's just finding his way in the world I think, I make sure he doesnt get anything nasty like bits of string or the like, but what can you do unless you carry him in your arms. Just gotta keep an eye on em, tidy up every second your not with him and pray for the best!!!