First,
try talking to them in a reasonable way (via e-mail).
Some will leave you alone if you object: they want easy resources but don’t want to be nasty about it. Few respond well to tirades or whininess.

Also, read the “Hunker Down” option in the question above and work on setting up your city so the attacker doesn’t get anything.

Whenever you see an attack coming (or are going to be away from the computer) make sure you aren’t leaving resources and gold around to be taken.

Queue up troops to get your resources down below your stores.
To reduce gold you could spend it on research or happiness or hire knights or lower taxes.
If the attacks continue, you need to keep gold out of your treasury:

Wait to collect your level up quest rewards until the last minute and consider switching to a levy economy (see the BUILDING YOUR CITY).