I have been running a rails app on a machine, which I can’t restart (no root access) or physically pull the plug on, just using the prepackaged rails server, WEBrick. But I quickly found out that it was not that easy to kill my app. I’d run something like script/server -d -p 25000 > /dev/null . 
Notice that -d puts the server in daemon mode and > /dev/nullsends all the standard out to /dev/null .
I tried to kill the app using top and then killing with the process ID, then tried pgrep and kill on the command line, to no avail.

Finally I found the (simple) solution that I’ll repost here. On the command line of the machine running the server:

pgrep ruby

This will give you the process id of the rails app (assuming you don’t have another ruby app running). Note: if pgrep is not available on your machine try using the command:

top

and look for the id of a process called ‘ruby’.

Now kill that process using the command:

kill -9 [id]

Where [id] is the process id given by pgrep.

Update:
It seems kill -9 may be jumping the gun and perhaps you ought to be safe and follow the advice I found elsewhere. 
I’ll reiterate his point here. After finding the process id using prgepissue the following commands waiting five seconds before trying the next one until the kill succeeds:

kill pid
kill pid
kill -INT [id]
kill -INT [id]
kill -KILL [id]
kill -KILL [id]

Notice that -KILL is the same as -9 .