If a debt is more than 25 cents in Canada, it's illegal to pay if off in pennies. One such rule is that an employee should never use their corporate credit card to receive cash advances. Cash advances come with high transaction fees, and interest begins to accrue immediately. And they usually carry a higher interest rate than normal transactions. Specify Allowable Charges.
Limit charges to only specific things, such as meals or gas.
This will help employees judge what's appropriate to charge and what's not. Also consider outlining your rules on items that aren't allowed. The more specific you are, the less room for abuse without recourse. Discipline could range anywhere from a written warning through suspension or termination. The key with credit cards is to be clear, consistent and organized.