If you're feeling especially adventurous, you could edit the source of the Grub bootmenu at
/boot/grub/grub.cfg
and delete the menu entries you don't need.
You will have to deliberately ignore the message at the top of the file which says
# DO NOT EDIT THIS FILE
You'll need to be root, and the the danger of making the system unbootable is high.
Make a copy of the file, eg grub.cfg0,
Study grub.cfg carefully, and make sure you understand what you're deleting before you do it. If you're NOT confident, don't do it.
The section relating to Memory Test is as follows in my Ubuntu 9.10
### BEGIN /etc/grub.d/20_memtest86+ ###
menuentry "Memory test (memtest86+)" {
linux16 /boot/memtest86+.bin
}
menuentry "Memory test (memtest86+, serial console 115200)" {
linux16 /boot/memtest86+.bin console=ttyS0,115200n8
}
### END /etc/grub.d/20_memtest86+ ###