Here's how to do this in v 3.00.90. In Admin CP go to Templates > Manage Templates > Tickets and select the 'submitticket' template. At about line 34, comment this block of code, using '<!--' and '-->':
Code:
<!--
<tr>
<td align="left" valign="top" class="row2" width="30%"><span class="<{if $posterror == true && $priorityid == ""}>redtext<{else}>smalltext<{/if}>"><{$language[tpriority]}></span></td>
<td><select name="priorityid" class="swiftselect" type="hidden">
<{foreach key=key value=item from=$ticketpriorities}>
<option value="<{$item[priorityid]}>" type="hidden" style="COLOR:<{$item[frcolorcode]}>;BACKGROUND:<{$item[bgcolorcode]}>;"<{if $tgroup[priorityid] == $item[priorityid]}> selected<{/if}>><{$item[title]}></option>
<{/foreach}>
</select></td>
</tr>
--> Then right below it, add the following line:
Code:
<input type="hidden" name="priorityid" value="1">
This will make sure the user cannot select the priority when entering a ticket via the web interface and set the default priority for such tickets to 1 (Low)