It reaches the nucleus by a process of tunneling. You can never completely stop it, but it reacts exponentially to you increasing a barrier.

An excited state isn't a barrier, it's a state with a higher energy than ground state. Several non-ground-state orbitals have a wavefunction whose amplitude drops to zero toward the centre.

An electron won't tunnel to a location where its wavefunction amplitude is zero.

Practical applications, a laser that explodes if you turn it off.