Q:

I'm going to invade Europa, any international interferences that could stop me?

So I have some payload which is going to be sent to Jupiter's moon Europa.
While my contract partner who is shipping my payload didn't specify strictly enough what the expected achievements of our collaboration are, I have loaded a quantity of sets of microbes and other lifeforms1 onto the ship which will be navigated in its last stage to crash into Europa. Not just if my mission is a success, making it for future missions hard to differentiate between those "immigrants" and the population, but maybe even one of my little soldiers has the fitting weapon set to raid and annihilate the whole population actually living on Europa.

If this would be a real case this would obvious be an psychopathic and raging mission with possibly disastrous outcome for the scientific community and any effort for the search of alien lifeforms.
But since space is by international conventions not claimable and so the "damage" is done to no ones property, are there any resolutions that would make me reliable for this or even force me into stopping my plans?

1 The kind of organisms which are expected to feel pretty well on Europa.

A:

TL;DR:
We've all agreed to not do that, but there's no Intergalactic Police stopping you. But you would likely be stopped by earth police forces.

Relevant Treaties and Organisations
UN
The UN's Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies has been signed by 89 countries.
I'll quote a few relevant parts.

Article VI
  (...)
  The activities of non-governmental entities in outer space, including the moon and other celestial bodies, shall require authorization and continuing supervision by the appropriate State Party to the Treaty.
  (...)

and

Article IX
  (...)
  A State Party to the Treaty which has reason to believe that an activity or experiment planned by another State Party in outer space, including the moon and other celestial bodies, would cause potentially harmful interference with activities in the peaceful exploration and use of outer space, including the moon and other celestial bodies, may request consultation concerning the activity or experiment.

Article IX roughly means "please don't do that", while article IV means that it's up to whichever party to the treaty feels obligated to take responsibility.
NASA
NASA has an Office of Planetary Protection. Its mission is as follows

The mission of the Office of Planetary Protection is to promote the responsible exploration of the solar system by implementing and developing efforts that protect the science, explored environments, and Earth. The objectives of planetary protection are several-fold and include:

Preserving our ability to study other worlds as they exist in their natural states;
Avoiding the biological contamination of explored environments that may obscure our ability to find life elsewhere – if it exists; and
To ensure that we take prudent precautions to protect Earth’s biosphere in case life does exist elsewhere.

But again, this seems to rely on treaties and cooperation, and seems to be lacking a way to enforce anything.

But realistically, you need to launch from somewhere on earth. Should the government that controls your launch site become aware of your plans, they would probably shut you down.
Even if the country itself might not take responsibility, it's very likely other countries might pressure it to do so.

