You can run as root;
and then change the lines that look like this;
title Red Hat Linux (2.4.18-0.12)
kernel /vmlinuz-2.4.18-0.12 ro root=/dev/hda3
to look like this
#title Red Hat Linux (2.4.18-0.12)
#kernel /vmlinuz-2.4.18-0.12 ro root=/dev/hda3
and just do like that for every kernel that you don't want to use.
To make some kernel the default, either place its section first or change the default entry number to the appropriate number (remember that it starts counting with 0) in the default=0
You can remove kernels that you don't want to use with the command already suggested by lazlow
but not before you boot your system with the kernel that you've choosed to use.