I had the same problem as you. What I ended up doing was hiding the option from the user using CSS.
It is a little bit of work but it will hide the priority drop down from the users and all tickets will be automatically submitted as low.
Log in as admin, click on "Look and Feel" at the top. Under the default skin you are using, click on the little page icon under the template column. It will take you to a page listing all of the templates being used.
Scroll to the bottom and select template "tck_submit_2.tpl" and it will open it in an edit window. Here is where you will put in a small bit of CSS to hide the Priority Drop Down.
Find -
<tr>
<td class='option2'><label for='priority'><font color="#FF0000">{$lang['priority']}</label></td>
<td class='row2'><select name='priority' id='priority'>{$priority_drop}</select></td>
</tr>
This is the table row that houses the drop down for selecting priorities.
In the first <tr> tag insert "style="display:none".
So it will look like this <tr style="display:none">
This will make the whole table row invisible.
Now the only way to change a priority is to do it in the admin area.
Hope it helps.