First,use conntrack to correctly identify your entries:
conntrack -L -s 172.16.1.45 -d 123.123.123.123
This should display any connections that came from the internal IP of 172.16.1.45 destined to 123.123.123.123
Once you have confirmed the connections shown are the ones you with to delete/reset,paste the following after the command from above:
conntrack [...]