The only way someone can 'steal' your bitcoins is if they somehow gain access to your private key. You should encrypt your private key with another pass-code in order to increase your security.
Online wallets notify you if someone has managed to access your account too.
Mhm, I guess my point though, is that websites can alert you if someone has accessed your account, but they can't actually prevent the person from spending them, or necessarily even tell if it's you that's spending them or not. Something like this could be applied to the watch only accounts on bockchain.info, but I don't know, maybe this would be useless feature.