Correct, assuming the Canadian Check is not drawn on a US Bank (sometimes they are), they are not subject to Regulation CC. Yes, you can deny to accept the check for deposit, which would be the safest course of action for the Bank because Canadian bank's may not be subject to the same return guidelines as US Banks.
K. 229.2(k) Check
1. Check is defined in section 602(7) of the EFA Act as a negotiable demand draft drawn on or payable through an office of a depository institution located in the United States, excluding noncash items. The regulation includes six categories of instruments within the definition of check.
10. The definition of check does not include an instrument payable in a foreign currency (i.e., other than in United States money as defined in 31 U.S.C. 5101) or a credit card draft (i.e., a sales draft used by a merchant or a draft generated by a bank as a result of a cash advance), or an ACH debit transfer.