On rare occasions you might want to prevent end users from closing tickets. The easiest way to do that is to remove the checkbox that allows the user to close a ticket. You can use some CSS to do that (though, be warned, this might need adjustment if you have a custom or non-standard theme).
Place the following in your theme’s styles.css file:
div.checkbox label {
display: none;
}
Note: this is not a SECURE method – a really determined user can still submit a ticket with the checkbox turned on via some injected html. All this does is make the checkbox invisible.