Stop Vesting, Return Unvested Options to Plan Reserve
To stop vesting an option grant and return the unvested options back to the pool, head to the Cap Table tab, click on Plans and Equity Awards, click on the Cert number for the particular grant, then click the button "Exercise & Termination".
On the Exercise screen, select 'Stop Vesting.' Set the date from when you want the options to stop vesting. This will return the unvested options back to pool and leave the vested options with the grantee.