Hi @Amber3, welcome to the community.
Are you on Android version 6 (Marshmallow) or 7 (Nougat)? You can find this under Settings > About Phone.
If you are on Marshmallow, there's no blocking option, but you can divert calls from a specific number to voice mail. This will act as an auto-reject.
To do this you need to have the number added in your contacts app. While editing the contact, press the options button (3 dots) and tick the box "All calls to voicemail".
If you are on Android 7 it is easier, in your contacts app press the options button (3 dots) and select "Block numbers".
