Steve,
Currently, TFRs will delete from your screen after they expire. The is a valid and expire date for each TFR. Some will have a short buffer from the actual expiration date, but they will automatically be removed from your screen after that time. You can go into menu, setup, alerts and warnings and select not to display or notify you of TFRs, it may not the safest thing but it can be done.
You could actually remove them for your SD card. Be Careful. To do this you would insert your SD card into your computer and open the "Data" Folder and delete the three below files. Do not remove or change anything else as there is sensitive date here needed to run your GPS. This will delete any TFR and Metar Data.
metars.bin
tfrs.xml.bin
tfrs.bin.processed