Problem: I needed to test some code that would auto-reconnect to a remote service, without being able to stop/restart the service.
Solution: tcpkill
For example:
apt-get install dsniff
sudo tcpkill -i eth0 port 27017 # no more outgoing connections to Mongodb
There must be some traffic on the connection for tcpkill to work. You can't kill an idle connection. killcx might be of use in this case.