How to stop a program running as daemon

Don't use kill -9! This command is meant to be used in some specific extreme cases only.

According to the man page (on my Solaris box):

DESCRIPTION
 The kill utility sends a signal to the process or  processes
 specified by each pid operand.

 For each pid operand, the kill utility will perform  actions
 equivalent to the kill(2) function called with the following
 arguments:

 1.  The value of the pid operand will be  used  as  the  pid
     argument.

 2.  The sig argument  is  the  value  specified  by  the  -s
     option,  the  -signal_name option, or the -signal_number
     option, or, if none of these options  is  specified,  by
     SIGTERM.

 The signaled process must belong to the current user  unless
 the user is the super-user.

When you don't specify any signal, kill will send SIGTERM (kill -15) to your process. There are more aggressive signals you can send that are less violent than SIGKILL (kill -9).

Why avoid kill -9?

SIGKILL is a very violent signal. It cannot be caught by the process, which means that the process that recieves it has to drop everything instantly and exit. It doesn't take the time to liberates resources it has locked (like network sockets or files) nor to inform other processes of exiting. Often, it will leave your machine in an unstable state. Drawing an analogy, you could say that killing a process with SIGKILL is as bad as turning off the machine with the Power button (as opposed to the shutdown command).

As a matter of facts, SIGKILL should be avoided as much as you can. Instead, as mentioned in the article, it's suggested that you try kill -2 and if that doesn't work kill -1.

I've seen people rush into sending SIGKILL all the time (even in daily clean up scripts!). I fight with my teammates daily about this. Please, don't use kill -9 blindly.

As it seems that this iperf process is not responding to your "termination" signal (the default signal sendby a kill <pid> command). It usually means that the process is crashed somehow or stuck in input/output accessing.

You just need to be more violent and send a "KILL" signal:

kill -9 <pid>

Or

kill -KILL <pid>

This will terminate the process without waiting for it to properly finish.