tcpkill is part of the dsniff package which have had seen the last work on it at 2010 and FTBFS on IPFire too. Debian have a patchset of 30 + patches for dsniff which makes it even harder to give it a try to build.
Two possible solutions for your question with on board tools.
Via lsof with port definition and the kill command. Example for port 80 can looks like this
kill -9 $( lsof -i:80 -t )
The ss command provides also an --kill option (-K) which attempts to forcible close sockets. Which should also work since it needs and enabled ‘CONFIG_INET_DIAG_DESTROY’ in the kernel → git.ipfire.org Git - ipfire-2.x.git/search .