How to have a cell calculate only once

I have cell N29 calculating some values based on streaming data. Once a condition is met in cell Q29, the calculation or final value should be passed to cell O29 and no longer calculate in either cell.

For example: If cell Q29 = y then O29 should equal the last calculation of N29 (and no longer calculate) and N29 should be set to 0 and no longer calculate.