In conjuction with turning on the Submit Zero option under your Smartview options, you can simply enter either #Missing or #NumericZero into the data form in Smartview and click Submit. Both of these functions clear the data rather than replace it with a zero.
I often use this in conjunction with an IF statement (for both data forms and functions) so if a value previously submitted is now zero in Excel, it will be cleared on any subequently submission. eg. =IF(A1=0,"#NumericZero",A1)