I would be very weary of punishing a dog for being excited to see me! not exciting the dog when greeting them and tossing high value treats down worked for me
the part of the chest you will hit is a extremely painful place

my mother accidentally kneed Twiggy once for jumping up she went right back to it again the next day

punishment only fixes the āsymptomsā not the cause so chances of them going right back to what they where doing before at some point is quite high! unless you ātraumatiseā them, which is whether good or bad what punishment training does

and they still can quite easily go back to doing it
The dog doesn't interpret it as you not wanting to see him, but that you don't want to see him in that way.
It works really well if it is done
consistently. If only one person of the family does it one time it's probably not going to do anything. Everyone will need to do it at least once including a couple of "strangers" so the dog learns that it's not just a thing only you don't want him to do.
From what I've seen in your posts, your spaniel is still a young pup. With him you would want to just grab under then chin and shove him backwards while saying "no" or "off". I'm sure on a young growing puppy you could hurt them by kneeing them if you did it hard enough. And young puppies generally are small enough that anyone would be capable of flipping them over just using their hand.
I don't know if you've seen young large breed dogs play fight but flipping them a few times with your knee is nothing compared to what they are willing to do to themselves while playing. I've seen dogs slam right into all sorts of things and it doesn't phase them at all.
The big problem with treats is you won't always have treats around. The neighbor's nutty spaniel that they "trained" only listens if they're holding a treat in their hand. Not safe if your dog is about to jump up on a two year old or run out in a busy street.
You don't get respect out of a dog if you haven't spoken to them in their language before. They understand getting whipped once and being firmly scolded. They never get that with treats.
And spaniels need a firm hand. They're very independent which is ok but they need to learn that you're still in control. There will come a time when they won't have a care in the world if you have treats or not, they just want to chase the neighbors cat or chicken or run across the busy street to see that kid. You could have a whole bag of treats and he might not care until he's done with whatever he wanted to do.
I've heard countless times that if you discipline your dog you can't possibly love them but it's exactly the same with a child, if they don't get punished and know you mean it they aren't going to respect your commands.
That's why young dogs in their difficult stage benefit greatly from some physical punishment. If they've done something very bad that they should have known better about, and if it's done intelligently and just enough to where you know they've got your point you'll never have an issue.
They need to learn if they don't listen you will make them,
especially with hunting dogs. If they're on a retrieve 200 yds out where you really can't get to them, and they come across something dangerous, whether it's an animal or situation, they have to listen immediately without hesitation.
You just can't get that if they don't have a bit of fear that you might decide to drive all the way out there and make them.
This can be taught at a short distance and it carries over to long distances.
I'm sure the same would be applicable in a dog park or anywhere like that.