Sure.
Maybe see this, in case you need it:
[HowTo] Resolve an unbootable Manjaro (black screen, login failed, cant boot, etc)
It slightly depends which packages you installed exactly.
You can check with:
pacman -Qs optimus-manager
But I will assume you wanted a GUI/Tray thing, and since you are on KDE, I will assume you got the optimus-manager-qt package. So a removal, including all dependencies no longer required, and extra save files, would look like this:
sudo pacman -Rns optimus-manager-qt
If you installed other packages too or made any other manual changes to system configuration files, or other such things, you will have to handle that as well of course. Reference your original guide.
PS: You can also reboot from tty using:
systemctl reboot
PPS: These things might be helpful to you:
https://wiki.manjaro.org/index.php/Main_Page#Software_Management
https://wiki.archlinux.org/title/Pacman/Rosetta