Forwarding BTC without approval Since all I need in order to send a transaction is the receiver address.
What is the mechanism  (if any) that stops me to send transactions to someone BTC without his approval?
Since all I need in order to send a transaction is the receiver
  address.


You also need a private key. In order to send someone coins from your wallet, you need a signed transaction with your private key.


  What is the mechanism (if any) that stops me to send transactions to
  someone BTC without his approval?


You don't need an approval of someone to send him transaction.