MSPM0G1505: Clearing the SPI TX FIFO

If I use the TX FIFO in the SPI peripheral (operating in peripheral mode), what's the correct way to clear unwanted data from the FIFO?  If I load it with data in readiness for a controller's request which never comes, how can I change the data at a later point?  Must I reset the entire SPI peripheral, or is there an option to clear out the FIFO?