How do I turn off the ability for scammers to add me to other fake telegram channels?

Go to your Settings > Privacy and Security > Groups & channels > Choose My Contacts OR Choose who to always allow or never allow. See the screenshots below for a more detailed explanation.

Step 1: In your Telegram account, click on the hamburger button and choose Settings

Step 2: In Settings, choose Privacy and Security

Step 3: In Privacy and Security, choose Groups and Channels

Step 4: Choose who to allow to add you to groups and channels. In this example, only the users contacts can add them to group and channels. Click SAVE.

For more details on this, visit Telegram’s Official FAQ page here https://telegram.org/faq