Zeros inserted in QAM Constellation
As others have mentioned, it sounds like you are talking about the null carriers in an OFDM system:



These bins are used to mitigate leakage from adjacent RF bands 2. Perhaps your tutor meant ISI in the sense that if adjacent RF bands were to spill into your OFDM signal, you would have some interference with your symbols on the edge of your bandwidth, but this is not ISI this is just, well, I (interference). The null carrier at zero is inserted because this carrier will experience significant interference in direct digital down conversion architectures (i.e. cannot recover modulated data when brought to DC at baseband).

In your case all the red/blue are QAM modulated data (if you use QAM for your pilot) the rest are left zero. The frequency spectrum would be exactly what you see in the figure above. It would be the same overall bandwidth, but with a fraction of the bandwidth used. It can be confusing because we use the term "guard band" when we mean frequency guard band which is different than what we mean by the cyclic prefix time domain "guard band" which mitigates the effects of ISI.