Hi fixthisnow,
This is because the iPad's standard Mail application doesn't have the ability to mark them as spam, they can mark them as trash but not spam.
I'd recommend logging into your Telstra Mail Migration Tool here https://emailupdate.telstra.com/consumeremail/home where you can set when you would like your Bigpond email account migrated to the new platform, which has an improved spam management system to help reduce this.
- Jarred