SSH force usage of a specific key

To force usage of a specific key, put in your ~/.ssh/config file :

Host [HOST_NAME]
   Hostname [HOST_NAME]
   User [USER_NAME]
   Port [PORT]
   IdentityFile ~/.ssh/[your_private_key]

Example for gitlab:
Host gitlab.company.com
   Hostname gitlab.company.com
   User git
   Port 10022
   IdentityFile ~/.ssh/[your_private_key]