I've many times heard this phrase:
It wasn't me
Here the case of me is accusative. In that case should we say
It wasn't us
if the agent is plural?
In short, yes. The following exchange would be correct.
Teacher walks into a classroom full of students and there are drawings on the board
Teacher: Did you guys do this?
Students: It wasn't us (who did that)!
The "who did that" part there is in parenthesis because in regular speech, nobody would say that. It is correct, but in regular speech, that part is simply implied.
The subject of the sentence is "it", and so the main verb has to be "wasn't" not "weren't".
Yes, it wasn't us is correct.
Strictly speaking, English follows Latin in using the nominative case rather than the accusative case after verbs of being. This is logical, since the accusative case - in this instance me or us - indicates the person/object which the verb affects. Eg I hit him.
With verbs of being, however, there is no object acted upon, so it takes a complement in the nominative case - I, we, he etc (not dative as stated in the questiom), since they are the same thing.
"Is that you?" "Yes, it is I." "Can I speak to Sue?" "This is she."
In practice, these forms sound strange in English, although entirely logical and normal in Latin, so we tend to say "Yes, it's me!", which sounds right but is grammatically incorrect, as me cannot be the object of is.
It wasn't us is not grammatically correct. The 'us' should be 'we', since wasn't is a linking verb, making the 'us' a predicate nominative, so it should be 'we'. It wasn't we, just as It wasn't I. Our common usage aside, 'we' is correct.