If someone gets hold of my wallet and also learns my seed and passcode, can I do anything to stop my wallet being drained?
No you cannot. If someone has access to your seed, they can recreate your wallet. If they have access to your wallet and password, they can also spend coins or obtain your seed.
A hardware wallet can add additional security by requiring physical access to the hardware wallet, but if the seed is ever compromised you are still out of luck so protect the seed at all costs.