Note that the library's JavaScript file is called sorttable (two Ts), but the class you add to the table is sortable (one T).

If you don't want the bottom row of your table, then add it to a section in your table (which is what you should be doing anyway, according to the HTML spec).

If you want to show "sort icons" before the table is sorted, add

table.sortable th:not(.sorttable_sorted):not(.sorttable_sorted_reverse):not(.sorttable_nosort):after { 
    content: " \25B4\25BE" 
}

If you'd like some column headers to not be clickable, meaning that your users won't be able to resort the table by those columns, then add class="sorttable_nosort" to the column header.