How to make selected option disabled in ruby

This is my select code the options i had selected i want to make it disabled. how to do so?

<td> <%= select("gu", "rm_id", Rm.where(category_id: Admin.select("category_id").where(id: current_user.id)).collect {|p| [ p.name, p.id ] }, {prompt: 'Assign RM'},{onchange: "update_rm_div(this.value,#{gu.id})"}) %></td>

Any help is appreciatable