SQL Server: what should be done when a process is blocking another process?
Despite this question having already being answered and the answer accepted, I'm going to try and make a case for the contrary approach... 

DO NOT EVER BLINDLY KILL A SPID

Unless you know what work that SPID has done, you have no understanding of the scale of the rollback you're about to initiate.

The worst kind of blocking chains are those which have unexpectedly brought your 24/7 system to its knees. 99% of the time, the query at the head of that blocking chain is going to be a plain old select query gone awry, queueing up any and all write activity behind it. In these circumstances, a KILL will save the day.

The other 1% of the time, that rogue query will be the last step in a long running transaction that will take as long to rollback as it did to get to the stage it's at now. Taking a moment to understand how you got here will 99% of the time be unnecessary. 1% of the time, you could be saving minutes/hours/days of downtime.