Assumpta, I forgot to add, distinguish between normal misdemeanours and big no-nos.
Normal bad behaviour - jumping on people, digging holes where not allowed etc, an ignore, a quick 'no' or 'uh uh' in a gruff voice works.
For big no-nos, such as resource guarding, aggression to humans or dogs you need to be very firm and nip the behaviour in the bud. A 'no' may not be sufficient deterrent. In your case, your trainer assessed what would work best. And you have the desired result, a dog that is not aggressive to other dogs.
Fwiw, if my male dog behaved aggressively towards another dog when I have said 'no, enough' he would get a smack as well. Not in anger, but in a calm way. The dog understands it as a signal of my being very displeased with him and adjusts his behaviour accordingly. Short, clear, effective correction and problem over.
So, IMO,
Ignore behaviour which is undesirable attention seeking (e.g., jumping) Reward good behaviour with praise.
Say 'no' or 'uh-uh' for normal undesirable behaviour (e.g., digging holes in the garden) and reward dog when they stop doing the undesired behaviour.
Be very firm with big no-no behaviours, whether it is resource guarding or aggression towards humans or other dogs. I would say 'no' sharply and follow with a physical correction if necessary. This behaviour should be nipped right away and not be allowed to continue as it is very hard to unlearn aggression, especially if the dog sees it is as paying off. Please do not ignore this behaviour, it will not disappear on its own. Nor should you get stressed and hysterical about it. Dog will sense you are not in control and will get stressed as well.
And as always, reward good behaviour.
Best of luck. Honestly this is not rocket science. Most dogs want to please their owners. And if you set very clear expectations, the dog will happily try to do the right thing.