hello Dan and welcome to the 4images forum,
Example for default templates
1.)
for remove the button
open template
/templates/default/details.html
delete tag
{lightbox_button}
and
open template
/templates/default/thumbnail_bit.html
delete tag
{lightbox_button}
2.)
for remove the link when user logged in
open template
/templates/default/user_logininfo.html
and delete this line
» <a href="{url_lightbox}">{lang_lightbox}</a><br />
4.)
to go 100% sure that no1 is using lightbox, delete lightbox.php where you config.php is