You have the ability to favorite writers. Any favorited writer will be automatically added to your account so you can send them assignments any time. Hence the name, "save for later."
Additionally, you have the ability to block applicants by clicking the reject icon. This will prevent the writer from ever showing up in your casting calls again.
You can also favorite previous applicants from the Manage > Users section. Here you can see all of your previous applicants and who you have favorited.