I'm not sure how this issue can be resolved since it applies to both xscreensaver and gnome-screensaver, but effectively there is no way of disabling the "webcollage" hack short of deleting it it manually. The "webcollage" hack has the potential to display porn. See the following bug discussion:
Please supply a simple and intuitive way to disable offensive screensavers while still being able to use the xscreensaver hacks.
No. We provide for convenience a script that by default creates .desktop files necessary for gnome-screensaver out of ALL of what's available in xscreensaver.
If you don't want a *.desktop file for webcollage, simply don't create it.
If you insist, you can try adding a --no-offensive option to the script found in /usr/share/doc/gnome-screensaver-*/migrate-xscreensaver-config.sh* and contributing that, so that it would be easy to migrate all xscreensavers for use by gnome-screensaver.
Closing wontfix per comment #1, please reopen if there is some patch we can apply to the conversion script.