is it forbidden to delete wallet after each transaction?
It is not forbidden. Do remember to write down your seed password (make sure you triple check the spelling) or you won’t be able to access your wallet after you delete it. Bear in mind that restoring the wallet can take quite a long time since it will have to resync the whole blockchain and find your transactions. (this process will become a lot easier when the light wallet is released).
To restore the wallet, click “Add Wallet” select “Restore” and type in your recovery phrase.