Is it the space under the submit button or the space under the form (which is the problem I normally have)?
Try sticking the form in a table and hiding the form tags between the row and table tags. This will get rid of the space below the form issue....
<table>
<form name="form1" method="post" action="">
<tr>
<td>All the stuff in your form</td>
</tr>
</form>
</table>
Or, if it really is the space below the submit button, try using an image instead?
<input name="imageField" type="image" src="image.gif" width="235" height="20" border="0">