How can I kill a process using top on OS X?

# top

press shift + s. This will bring up:

signal [TERM]:

At this point you can just press return for a 'polite' process end request ("signal 15") that will let the process clean up files, release memory, etc. For process that can't be killed this way, you can enter the word ‘kill’ which is signal "9" then press return.

Now top prompts for a pid (process id):

pid:

Enter the pid of the process you want to kill and that should take care of it.

From the top man page:

  S<signal><pid>
          Send  <sig>  to  <pid>.   <sig> can be specified either as a number or
          as a name (for example, HUP).  The default signal starts out as TERM.
          Each time a signal is  successfully  sent,  the default signal is updated
          to be that signal.  <pid> is a process id.