It's depend on what you exactly mean under deleting of all rows. The method GridUnload could be very helpful in many cases, but it delete more as only grid contain.

Another method used intern in jqGrid is:

var trf = $(#list tbody:first tr:first)[0];

$(#list tbody:first).empty().append(trf);

Probably it is what you need. It delete all grid rows excepting of the first one. You can overwrite the code also as the following

var myGrid = $(#list); // the variable you probably have already somewhere

var gridBody = myGrid.children(tbody);

var firstRow = gridBody.children(tr.jqgfirstrow);

gridBody.empty().append(firstRow);