Welcome to Android Central! You may have to pay extra to install the Call Detector app: Straight Talk - Extras. From what I read, you can't block for free on Straight Talk.
It's not advisable to use your email address as your username, for your own security.
There are also free blocking apps that work on Straight Talk - I used There are also free blocking apps that work on Straight Talk - I used BlackList for a few years on ST.