Open Telegram
Go to Contacts
Select the contact
Tap the contact name
Tap Edit
Tap Delete Contact
Confirm deletion
If the contact still appears, delete the chat and clear synced contacts in Telegram settings
On Android, go to Settings
Tap Privacy and Security
Tap Data Settings
Turn off Sync Contacts
Tap Delete Synced Contacts
On iPhone, go to Settings
Tap Privacy and Security
Tap Data Settings
Turn off Sync Contacts
Tap Delete Synced Contacts