Yes you can flash the N900 with the Maemo Flasher to have only Maemo 5 installed.For that please read the Wiki on Updating the Firmware.

http://wiki.maemo.org/Updating_the_firmware

Also you can remove Gingerbread and still keep Maemo 5 without flashing the EMMC.Please read the Wiki on Uninstalling NITDroid.

http://wiki.maemo.org/Nitdroid_easy_install_on_EMMC

Next time please use the power search before you post something like this.