The following CSS will hide the expand/collapse images for treenode.
CSS:
<style type="text/css">
.rtPlus
{
display: none !important;
}
.rtMinus
{
display: none !important;
}
</style>
You can also hide the expand/collapse images from client side. In page load event access the RadTreeview and loop through each node and hide the node. Please refer the following forum post which demonstrates the same. Disabling node collapse at the client