You may simply use a combination of explode(), substr() and implode() with a simple Loop to achieve that (if you prefer not to use Regex - that is...). The Snippet below (which you may Quick-Test Here) demonstrates how:

<?php

    $html = "

                <p> something </p>

                <select name='bla' id='bla' class='REMOVE'>
                <option></option>
                <option></option>
                <option></option>
                <option></option>
                </select>

                <p> something </p>

                <p> something </p>

                ";

        $arrParts   = explode("</select>", $html);
        foreach($arrParts as &$part){
            if($selPos = strpos($part, "<select")){
                $part = substr($part, 0, $selPos);
            }
        }
        $html = implode("", $arrParts);
        var_dump($html);