Can a Canadian eTA be cancelled?

Your eTA cannot be cancelled once it has been submitted, so you will not be able to get a refund even if you do not go ahead and use your eTA. If you are trying to cancel your eTA because you have completed some incorrect information on your application, it may be possible to update it depending on which information you need to change.
Once you have received email notification of your application being submitted, you can use the reference to try and update the details without the need to reapply. However, certain fields cannot be updated and if your application is refused due to the incorrect information, you will need to send another application and pay the fee again.
If you wanted to cancel your application because you realised you do not fit the eligibility criteria then the situation is still the same and you cannot cancel your application once you have submitted it as it will already be getting processed.