First, you have to enable quota on your system. Have a look at the quota section of the appropriate "Perfect Setup" tutorial for your distribution on HowtoForge.

After you've enabled quota on your system:

To set quotas for a user or group, use the setquota command (see

man setquota

for more information), e.g.:

setquota -g $group $blocks $blocks 0 0 -a
setquota -u $username $blocks $blocks 0 0 -a

Replace $group, $username and $blocks. Note: $blocks is the quota size in MB * 1024! To deactivate quotas for a user or group, run these commands and set $blocks to 0.