If you want a button that just resets the form (but won't reset the captcha image):
Insert > Form > Button - and in the Properties Panel/Inspector set the action to 'Rest Form'
Make sure that the button is inside the form tags.
If you want a button to refresh your form page (so that the form resets and the captcha image changes):
Insert a button - Insert > Form > Button
In Properties Panel/Inspector, for 'Action' select 'None' - give it a value, i.e 'Reset'.
Go to Window > Behaviors and in the Behaviors Panel, click the plus sign and from the options select 'Go to URL'. Enter 'contactus.php' for the URL.