Who owns the email address nobody@ns1.roachhost.com

I OWN IT.  WHY DO YOU ASK?

No one owns it.  Its a blank email most likely there for testing purposes.

In my opinion, the email would be read (if at all) by the server
administrator at roachhost.com, although it might not be so innocent:

Scroll down to the text in yellow.
http://72.14.203.104/search?q=cache:46l7ITYddIUJ:www.extropia.com/tutorials/security/index.html+%22nobody%40*.com%22+administrator&hl=en&gl=us&ct=clnk&cd=1

Computer roachhost.com has been found. Visual IP Trace has drawn
information from its own database and from information specified by
the owners of roachhost.com. Both sources tend to agree that the host
is located around Denver, CO, USA.

This system is a web and file transfer server 
The following copyright notice appears on the website, which could
give a clue to the website creators: © 2004 Roach Host

 Network Contact Information: The following details refer to the
network that the system is on.
   OptinRealBig.com
   1333 W 120th Ave. Suite 101 Westminster CO 80234 US


These people own the domain roachhost.com (see below)
  Domain Contact Information: The following details refer to a name
registered for this address.
   optinrealbig.com llc
   scott richter (srich10195@aol.com) Phone: +1.3035509828 Fax:
+1.5555555555 Address: 1333 w 120th ave suite 101 westminster, co
80234 US

If you contact Scott Richter (above),ask him to find out who user nobody@ns1 
on his domain is. I don't know if he will give you the information due
to privacy rules but if this person is spamming you, you can complain
to Scott and ask Scott to remove their email access to his domain.
This is the best anyone can do.

This can be answered in several steps.

1) The domain roachhost.com is essentially all that is required from
the domain point of view to direct email.  The fact that it is
prefixed by ns1 implies that you may be dealing with a Name Server
(and not a mailserver).  Name Servers deal only with translating human
addresses like roachhost.com and converting it to an IP address that
the internet is based upon.

2) Running a WHOIS query on roachhost.com brings the following information:
   Domain Name: ROACHHOST.COM
   Registrar: ENOM, INC.
   Whois Server: whois.enom.com
   Referral URL: http://www.enom.com
   Name Server: NS2.ROACHHOST.COM
   Name Server: NS1.ROACHHOST.COM
   Status: REGISTRAR-LOCK
   Updated Date: 30-dec-2005
   Creation Date: 14-jun-2005
   Expiration Date: 14-jun-2007

This confirms what I mentioned about the prefix 'ns1'.

3) The 'nobody' part of the email address is usually reserved for
email that cannot be matched to a user.  If the user does not exist
then it is usually forwarded to the user 'nobody', which is a default
user in UNIX/Linux Operating Systems (which a majority of email
servers or run on).  This can be configured to collect / disregard
email that arrives and cannot be matched to a user.

4) Given the email address that you have submitted, I can safely
assume that you will get absolutely nowhere by sending an email to it.
 The email address looks as if it may be the originating address
(which can be programmed to be anything) for spam email.  The most you
could possibly do is to contact somebody at www.roachhost.com and
explain your problem to them.  Failing that, the information from the
WHOIS database shows the owner of the domain as:
   optinrealbig.com llc
   scott richter (srich10195@aol.com)
   +1.3035509828
   Fax: +1.5555555555
   1333 w 120th ave
   suite 101
   westminster, co 80234
   US

Hope this is of help.