Your checkForm function should have the code to validate as well as submit. Your button should be of the type "button" and not submit. If the validation fails make an early exit from the function.
Add the following code in the $$HTMLHead field:
Add this code for a submit button as passthrough HTML or with the HTML style:
<INPUT TYPE="Button" VALUE="Submit" onClick="checkForm(0)">
Now that you've added a special button, you'll have to remove the default one that Domino generates via the FAQ on how to remove the submit button.