Don't know if there is something built in as jQuery draggable has options for this, but couldn't find anything similar for gridster.
You could always create the functionality yourself:
$(".gridster ul").gridster({
widget_margins: [5, 5],
widget_base_dimensions: [100, 100]
}).on({
mousedown: function(e) {
$(this).data({top: e.pageX, left: e.pageY});
},
mouseup: function(e) {
var top = e.pageX,
left = e.pageY,
ptop = $(this).data('top'),
pleft = $(this).data('left');
$(this).data('dragged', Math.abs(top - ptop) > 15 || Math.abs(left - pleft) > 15);
},
click: function(e) {
if ( $(this).data('dragged') ) e.preventDefault();
}
}, 'a');