There are several problems here (are you sure you wouldn't be better off using auto logoff?):
*/1 * * * * pkill sshd
The "/1" should be "/10" to run every ten minutes.
There is no user specification: I suspect that you'll find some message to the tune of "unknown user pkill" in the syslog.
It's to be seen whether
pkill is in cron's PATH when it runs.
All in all, I'd try with:
*/10 * * * * root /path/to/pkill sshd
Then you have the problem of sshd restarting after having been killed, as D'Arcy Nader pointed out in his answer (but that's probably all right, otherwise you wouldn't want to do serial killing through cron).