How would this be said?

This seems like a response to a question. We'd have to know the question, so that we could use the right verb in response. For example:

An mbeidh tú ansin?
Ní bheidh

An íosfaidh tú é?
Ní íosfaidh

srl.

Thanks grand. I was actually thinking of the phrase:

They can't, nor won't

The "can't" part is easy:

Ní féidir liom (leat, leis, léi, srl.)

The "won't" part requires the negative future of the verb used in the question. For instance:

An ndéanfaidh tú é?

Ní féidir liom ná ní dhéanfaidh mé

or

An íosfaidh tú é?

Ní féidir liom ná ní íosfaidh mé

As a point of logic, however, I'd never say such a thing, since if I "can't" do a thing, then whether I will or not seems beside the point.