Using państwo and surnames.

How would I say Mr and Mrs Kowalczyk using the word państwo?
Would it be państwo Kowalczykowie?
And if I wanted to say Mr and Mrs Kowalczyk aren't here would I say:
Nie ma państwa Kowalczyków?

yes - I guess you just wanted to get rid of any doubt - anyway you got good ear for Polish

(both of the examples you've given were correct)

probably this word order would be better:
Państwa Kowalczyków nie ma [w domu],

but it depends on the conversation context, for exemple, if you want to say that most people arrived at a meeting/gathering with the exception of Mr and Mrs Kowalczyk, then the word order suggested by you is more natural:

(Przyszli prawie wszyscy: państwo Nowakowie, Kowalscy, Wiśniewscy, nie ma [tylko] [państwa] Kowalczyków i Woźniaków.)

Thanks for your input.

Of course, you usually don't actually say państwo in the third person, especially if the context makes it clear you're talking about a married couple: "Przyszli prawie wszyscy: Nowakowie, Kowalscy, Wiśniewscy, nie ma tylko Kowalczyków i Woźniaków."

Sounds more natural to me (though I'm not a native speaker).

However, you have to remember that it ain't work with everything.
Some names ain't change when it comes to PRZYPADKI.

Even in this case you can say: Nie ma państwa Kowalczyk.

It is very difficult subject and many Poles also have problem with that.
I wish you good luck in learning!

B.u.l.l.s.h.i.t. You can't say that.

LOL. 90% of Poles would say it like that. And I'm one of them. Nie ma państwa Kowalczyków sounds wierd. You can say Nie ma Kowalczyków, or Nie ma państwa Kowalczyk.