Una y no más Santo Tomás
 

How can I say in English?
Una y no más Sto Tomás.

Once is enough?

Your proposal works, but I see the expression used in a simpler fashion.

one time only
only one time
only once
once only
once and never again
today only (a one-time event)
the first and the last time

I'm only going to explain/do this once.

Update: Here is one more translation from our idioms section.

Looking around the internet, I see that some have suggested a couple of refrains, like 'once bitten, twice shy' and 'fool me once, shame on you; fool me twice, shame on me'. These both mean 'once is enough', as in, you don't want to experience something twice (especially said of a painful/hurtful event).

yes the context was when you go to a restaurant and the food doesn't rise to your standards and you say una y no más Santo Tomás.

And your guess of once is enough is the best choice in that case.

Given the case of sub-standard food from a restaurant, another thing you could say is, "I've had my fill (of that place)."