Learn English – Should there be a comma after “Thank you”

Not really. There are many cases for which a comma is unnecessary.

He is strong, and therefore a likely candidate.

However strong he is, he is not a likely candidate.

And so on.

As for subordinate clauses, where the words function as subordinating conjunctions, the comma isn't always necessary either.

He is strong, therefore he must be a likely candidate.

Still, I would use one with however in that case:

He is strong; however, I don't think he's a likely candidate.

Note the semicolon. A period would work there as well. This is because "however" doesn't really work as a subordinating conjunction. Therefore isn't usually classed as such either, but I believe it can function as one just fine.

I see there are two comments and I read them as being somewhat contradictory. Therefore I'll give an answer. If we are reporting dialog then there are several options, here are three.

1. "No thank you."

2. "No, thank you."

3. "No. Thank you."

The first, as indicated by WS2, is an idiom and is spoken smoothly without a pause.

The second indicates a minor pause.

The third indicates a more pronounced pause with the 'Thank you' possibly coming as an afterthought.