Welcome, PowerInside!
Like all things linux, there is more than one way to accomplish this...
1) If you are using persistently saved changes, then you can uninstall packages using the 'removepkg' tool (this is a standard slackware package management tool) -- this won't modify the porteus base module containing the package -- the package will loaded at bootup and then removed from your live filesystem when your saved changes are applied.
2) To remove the selected package entirely from a base module, use the 'rempkg' tool, which is a porteus specific app that brokenman put together. Just type 'rempkg' in a console and press enter, and a GUI dialog will open up prompting you to select a module to alter, and then you can select packages to remove from that module. The module will be unpacked, modified to remove the packages, and repacked into a new module, which you'll need to put in your /porteus/modules folder to replace the stock module (I'd suggest keeping a backup of the stock module just in case something goes wrong -- it can go in /porteus/optional).
3) If you're just looking to hide an entry from showing up in the menu without messing with packages, you can add a line to the file /etc/rc.d/rc.local, which will delete the .desktop entry before kde/lxde starts up. The line would look something like 'rm /usr/share/applications/kde/kopete.desktop' (without the quotes). More on using rc.local here:
http://porteus.org/info/docs/47-porteus ... local.html
If you're not saving your changes persistently, you can place your modified rc.local in your rootcopy directory (i.e. /mnt/sdXY/porteus/rootcopy/etc/rc.d/rc.local) and it will be applied to your system, even in always fresh mode.