eliminitaing spikes from the circuit input
You did not specify how large your input signal is and how large are the "spikes and high voltages".  If your input signal is below say 0.5 volt peak, then you can use back-to-back diodes connected from your input to ground.  The diodes will not conduct for the signal and thus appear as high impedance devices but will conduct for anything above about 0.7 volts.  If your signal is above 0.5 volt, you can bias the diodes to compensate.  For very fast spikes, as already pointed out, you may need a device designed just for that such as a transient voltage suppressor or even a spark gap.  You really need to specify your voltage levels and the source impedance of the signal and the interfering signals before a more definitive answer can be given.