Thread: Puppy biting older dog

We recently welcomed "lulu" into our pack. She was a rescue pup through Poundpaws from a pig dog litter that was dumped.
She's a cool wee character, was bottle fed from 5 days old so she loves people.
She's full of beans and old mate who dumped them would probably be kicking himself if he saw her now and the instinct she's showing.
Problem is, said instinct involves her getting along side Bo and latching onto his face/neck/behind the ears.
She's only playing but Bo is so patient and laid back and her little puppy teeth are like needles so she's caused him quite a lot of damage, drawing blood and giving him little injuries all over his face and neck. He just takes it and won't tell her off and it's starting to do our heads in. We constantly tell her no when we see her doing it and she's had a few whacks on the nose when we've been able to catch her in the act. Trouble is, she's also super smart so instead of stopping it, she's resorted to doing it only when we aren't looking.
She's too small to get a garmin on and I am desperate to knock this behavior on the head as our big beautiful boy is starting to look like he's got a serious Fentanyl problem or something.
Any suggestions?