You wouldn't need an interstellar object, there are plenty of local objects which can impact Earth too.

Interstellar objects, however, travel faster due to their hyperbolic excess and so would be more difficult to detect.

It would be possible to stop it if it is detected very early and we have a deflector mission ready on the launchpad, the idea behind asteroid deflection is you have a precise orbit for them and impact them early enough such that the small change in trajectory you make is enough to make it miss Earth completely.

In practise, this is probably not feasible.