I'm not quite sure what you mean. Mer isn't installed into Maemo, so you can simply remove or reformat the drive that Mer is on and you can use that drive for whatever you'd like after that.

You can remove the Mer installer from Maemo if you go into the Application Manager and remove it as you would any program.

You can remove the customized bootmenu by downloading the latest Maemo and flasher-3.0 utility and then flashing the original initfs to the tablet. To do this, make sure that flasher-3.0 is executable (you can type "chmod a+x flasher-3.0" from terminal) shut down your tablet and then hold down the home button while hitting the power button on the tablet until a little USB icon appears in the top right hand corner of the tablets screen. Then type this command in the terminal of linux (from the same directory that flasher-3.0 is downloaded to): sudo ./flasher-3.0 -f -F RX-44_DIABLO_5.2008.43-7_PR_COMBINED_MR0_ARM.bin --flash-only=initfs -R