Guineas very rarely bit out of aggression towards us. It is normally they are scared or, in the case of my Bumble, want the toilet and we didn't pick up on the fidgeting, so need to be told more forcefully to put them down please.
Do you or your family use any scented soap or strong chemicals or creams on your hands? The reason I ask is guineas have sensitive little noses and can easily pick up the scent of things that may irritate them, such as those ridiculous lavendar shavings shops sell - we might like it, they won't bless them.
How much human contact do they get? For example, do you just sit out with them while in the hutch, or is it all handling? I have found letting a guinea just be there with you helps a lot.
Does anyone ever feed them through their cage bars? This is the easiest way to get a nippy pig and is strictly forbidden in my house, I have had several families bring their guineas and moan they nip and I when I ask if the kids feed them through the bars, the answer is usually 'all the time'.
Another reason for nipping can be hand rearing - JoJo was hand reared and as he learnt from day one (his mother died) that hands/fingers came with milk/food, he liked to eat them. He has now thankfully stopped this habit but it has taken almost a year.
Punishing guineas as you would a horse for example (my horse would get a hefty smack if she ever bit anyone) won't work as such - you need to replicate their language. If a herd member bites another, the bitten one will run off if lower in the herd - which is what you have been doing. Obviously if higher in the herd a full scale war will erupt, which you don't want to be beating your guinea pigs obviously! But you can't back down or put them back, you have to perservere. I can't say if it definitely worked but I used to say 'oi!' in a gruff voice to JoJo, a sound he didn't like, and he didn't get any veggies at all or any treats while being handled for a long time too to break the association 'cuddles mean nomming'.
It will take a lot of time to break this habit as it sounds quite well established unfortunately. If you really regret having them, it may be best to find them a new home if you really feel that way. You being unhappy is not good for them as they will pick up on this, and cause everyone extra stress.
I am NOT meaning to imply you are a bad owner at all as you are clearly trying to solve the problem, but just want you to know there are people who will help you if you're struggling xx