I suspect that the rtrim solution would be quite efficient, but another way to do this would be:
$result3 = mysql_query("SELECT * FROM regions_keywords WHERE NewsRegionID = $RegionID");
$NewsKeyWords = array();
while ($row3 = mysql_fetch_array($result3)) {
$NewsKeyWords[] = $row3['NewsKeyWord'];
}
$Words .= implode('; ', $NewsKeyWords);