Add script tag.
Make onclick events return true.
Test for numerical values (sufficient, in my opinion).
<script language="javascript" type="text/javascript">
function set_hidden(inHdnVarID)
{
var oHdn = document.getElementById(inHdnVarID);
oHdn.value = 1;
}
</script>
<cfform name="myFrm" id="myFrm" action="RptPage.cfm" method="post">
<table>
<tr>
<td>
<input type="submit" name="sbtn_submit" id="sbtn_submit" value="rpt_1" onclick="set_hidden('i_hdn_rpt_1'); return true">
</td>
<td>
<input type="submit" name="sbtn_submit" id="sbtn_submit" value="rpt_2" onclick="set_hidden('i_hdn_rpt_2'); return true">
</td>
</tr>
</table>
<input type="hidden" name="i_hdn_rpt_1" id="i_hdn_rpt_1">
<input type="hidden" name="i_hdn_rpt_2" id="i_hdn_rpt_2">
</cfform>
<cfif isNumeric(Form.i_hdn_rpt_1) >
<CFHEADER NAME="Content-Disposition" VALUE="inline; filename=rpt_1.csv">
<CFCONTENT TYPE="application/csv">rpt_1,1,1<cfabort>
</cfif>
<cfif isNumeric(Form.i_hdn_rpt_2)>
<CFHEADER NAME="Content-Disposition" VALUE="inline; filename=rpt_2.csv">
<CFCONTENT TYPE="application/csv">rpt_2,2,2<cfabort>
</cfif>