I'm pretty sure you can only do one at a time, or all of them at once. You should be able to click clear all field notes or something like that. I'm not sure if that's what you want though.
I am happy to delete all as it will save me a lot of time. I can chase the logs I need by following my GPS log as there is only about 30 of them to do.
How do you delete all field notes ??