The learned members have already given you all the tips in the above comments, which I am simply combining to give you an answer:
There is nothing wrong with passive voice when used appropriately. So you can safely ignore the style guides that 'warn against' using it. However, what sounds proper in active voice can become 'weak' if needlessly twisted around into passive voice. So you can 'keep it simple' and go with active voice whenever feasible.
In fact your statement seems weakened not by passive voice but by the double negative in "should not be treated without disgust." Multiple negatives are always harder to process cognitively, and can confuse the reader into misinterpreting the meaning as "should not be treated with disgust" or "should be treated without disgust" which is the opposite of your message.
If you want to 'strengthen' the above sentence, you can avoid the double negative and simply write "should be treated with disgust", as the member rightly advised in the above comment.
Moreover, the 'can' in 'he who can fool a customer' contributes an element of ambiguity, so I would advise you to write "he who would fool a customer" (which means 'he who would not hesitate to fool a customer') or you could simply write "he who fools a customer," which is the strongest statement.
He who can fool a customer should be treated with disgust. (medium-strong)
He who would fool a customer should be treated with disgust. (stronger)
He who fools a customer should be treated with disgust.(strongest statement)
I am of course aware that 'he who fools a customer' does not have exactly the same meaning as 'he who can/would fool a customer', but it does help you to avoid ambiguity and (thereby) further strengthens your sentence!