There was a cult that used this very verse to commit mass murder (and/or suicide, I believe). (Or it was one person who killed some people, I don't remember the specifics, but they cited that very verse as their justification for doing what they did.) Their reasoning was correct, but their application was wrong. You're making a similar mistake. Your application is correct, but your reasoning is wrong.
Paul is saying that he who is physically dead, as in, no longer alive on this earth, is freed from sin, and He uses that reality as an analogy for a SPIRITUAL truth, how we are dead to sin SPIRITUALLY because we are dead to the law.
Do you see how that changes the meaning?
The only way we can be freed from sin is by literally dying, because then we are no longer under the law of this world, and therefore there is no condemnation to those who are no longer under the law. But as far as I can tell, neither you nor I are actually dead yet (unless you're claiming to be a zombie, in which case you might want to get that looked at...), and so we are NOT freed from sinning, despite being dead to sin.
Paul tells us, a few verses later in verse 11, to "consider yourselves dead to sin." What he DOESN'T say is "Consider yourselves unable to sin."
He's saying that sin no longer has power over us, because we are dead to the law in Christ, but alive to God.
Therefore
do not let sin reign in your mortal body,
that you should obey it in its lusts.And do not present your members as instruments of unrighteousness to sin, but present yourselves to God as being alive from the dead, and your members as instruments of righteousness to God.For sin shall not have dominion over you, for you are not under law but under grace. - Romans 6:12-14
http://www.biblegateway.com/passage/?search=Romans6:12-14&version=NKJV
Verse 12 especially is, indirectly, telling us to resist sinning, not because you will always succeed, but because "in Christ we are more than conquerers," being no longer under the law, but under grace.
He doesn't say, "You will not sin any more," but rather "don't let sin have dominion over you." Sin will still be present, but don't let it rule your life.