No reduction after session of board - Exception

After the adjournment of the board each year, neither the governing body of the city nor the city board of equalization may change or alter any assessment. Neither may the governing body or the board of equalization reduce or abate, or authorize the reduction, abatement, or return, of Page No. 1 any taxes levied upon such assessments for any cause except that the property assessed was not subject to taxation at the time the assessment was made.