Client side validation is performed in the client browser using Javascript. The down fall of Client validation is that it can be disabled if javascript is turned off in the browser.
Since Server Side validation is performed on the php server, the client cannot get around it.
I recommend always using server side validation.
you can remove client side validation by opening the bindings panel, go to Windows -> Bindings.
In the tag selector, select the form tag, then in the bindings panel select each validation and click the remove button.
Also it seems like there is a way to make the validation process stop and revert back to the checkout form at each step of the way, rather than waiting until the final checkout button is selected and many errors are listed. How do I do that? 
this is done using client side validation, applied to the onBlur action of each form element.
