Is there any way and how to disable slideshow in xcgallery?
Or to disable only for guest users?

If not possible for guest,how to disable it for all users?

problem solved.
for those who need the same thing they can do it by removing this line:

<th align="center" valign="middle" class="navmenu" width="48">
<a href="<{$slideshow_tgt}>" title="<{$slideshow_title}>"><img src="images/slideshow.gif" width="16" height="16" border="0" align="middle" alt="<{$slideshow_title}>"/></a>
</th>

from xcgal/templates/xcgal_display.html
and all is okay after that.
Case closed!