if your /tmp is a directory in your / filesystem and you have nothing important in it, then just delete its contents... something along the lines of (as root):
rm -fr /tmp/*
rm -fr /tmp/.*
keep in mind that in the above example you'd still have problems if the / filesystem itself was too full, since /tmp resides on it...
wouldn't hurt to make sure the permissions are right too:
but if your /tmp is a tmpfs filesystem then rebooting would actually clear it automagically... you can find-out by looking at your /etc/fstab
file - feel free to post it also...