This is prob something you should have in your terms & conditions or a separate Non Disclosure Confidentiality Agreement. It may also be wise to have a summery sheet on any presentation just to re-enforce the point. There generally isn't any need to go too heavy handed with this as if a client thinks your up on your rights, they would be unlikely to try anything. Couple of useful links below.
I appreciate and understand to a certain extent IP law without the intricate understanding that a solicitor would. My concern is that how would you hold someone accountable before entering a contract with them.
In this particular scenario for example, I am pitching ideas to a Client in order to win their long term future business. No contract has been entered.
Basic Terms and Conditions should always be presented to any client. This will state many issues and one para will be the intellectual copyright and ownership of all presented work till payment. This is standard business practice and should be emailed or sent to client.