An electron spins about it's own axis. So it has rotational accln. Thus, according to Maxwell's Electromagnetic Theory , Rotational Kinetic energy must be released. Thus, at some pt. of time , shouldn't the energy of an electron become 0 & then shouldn't the electron move out of the atom, eventually leading to the atom only having the nucleus .
You are confusing macroscopic spin with the spin angular momentum of electron here. Electron is a point entity and does not posses a macroscopic spin (i.e. it does not spin on its axis). It does have angular momentum which is expressed as 'spin angular momentum' which is of-course quantized.
This link by John Rennie will be useful for understanding : The electron isn't spinning on it's own axis