If you have SSH access, you can manually edit the file /etc/csf/csf.deny with vim or emacs or your favorite text editor.
Here's a portion of the top of that file with some comments:
# The following IP addresses will be blocked in iptables
# One IP address per line
# CIDR addressing allowed with a quaded IP (e.g. 192.168.254.0/24)
# Only list IP addresses, not domain names (they will be ignored)
#
# Note: If you add the text "do not delete" to the comments of an entry then
# DENY_IP_LIMIT will ignore those entries and not remove them
Given that information, use a subnet calculator (such as this one from MXtoolbox) to determine the smaller subnet. Given your criteria, one possible subnet suggested by mxtoolbox would be 151.80.31.96/27, which would filter everything from 151.80.31.96 through 151.80.31.127.
Make sure that you restart CSF afterwards with csf -r
and then restart lfd with lfd -r
.