If you want to block some connections from specifically ip address and not use fire wall, you can add some parameters in the sqlnet.ora that can achieve  what you want.
 
for example:
/u02/oracle/product/9.2.0/network/admin/sqlnet.ora
# Generated by Oracle configuration tools.

NAMES.DEFAULT_DOMAIN = localdomain

NAMES.DIRECTORY_PATH= (TNSNAMES, ONAMES, HOSTNAME)
#This turns on the hostname/IP checking for your listeners
tcp.validnode_checking=yes
#invited nodes of the  ip address
tcp.invited_nodes=(192.168.11.89,192.168.11.3)

#exclude a list of clients
tcp.excluded_nodes=(192.168.11.100)

remember,you need restart listener.

Just a small tip -:)

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