Can 'nothing' ever be preceded by a plural verb?

I've had it drilled into my head that nothing is always singular, since it's essentially another way to say 'no thing'. However, in the following sentence, I'm having trouble ascertaining whether the preceding verb should be singular or plural:

There are nothing but liars in here.

Is this sentence acceptable? I'm inclined to believe that nothing is being used as an adverb rather than a pronoun, so it is correct to use 'are' rather than 'is'. Here's another example sentence:

They are nothing but petty thieves.

In this case, since the subject is simpler, it becomes more clear that the plural should be acceptable. Am I understanding this correctly?

The are applies to liars, not to nothing here.

Did you notice that you have but in both your examples? Nothing but as a phrase is used here in an idiomatic sense. We can reread the sentences as:

There are only liars in here.
They are just petty thieves.

That's how it is, I believe.

‘They are nothing but petty thieves’ is grammatical because, as others have said, they means those people the speaker is calling petty thieves. In the other sentence, ‘There are nothing but liars in here’, nothing but could, as in the first sentence, be replaced by just or only. But I wonder if it wouldn’t be more likely to occur as ‘There’s nothing but liars in here’. As noted in ‘The Cambridge Guide to English Usage’, there’s seems to be in the process of becoming a fixed phrase, regardless of whether the context is singular or plural.