To unshare asset you have to follow same process as you did in share asset, just one change - you have to mention only source publication id.
<asset:share name="publishAsset" publist='<%=source_pubid %>' /><%
<asset:load name="assetObjName" type="<%=assetType %>" objectid="<%=assetId %>" />
<asset:sites name="assetObjName" list="siteNameList" />
<ics:listloop listname="siteNameList">
<ics:listget listname="siteNameList" fieldname="id" output="site_id" />
<asset:removesite name="assetObjName" pubid='<%=ics.GetVar("site_id") %>' />
</ics:listloop>