This is an interesting question in the field of so-called “space law”, which is to say that there is essentially no formal jurisdiction in space beyond Earth orbit nor any means of enforcement. However, presuming you are a citizen of the United States and your operations are dictated from or by an entity incorporated or otherwise established (e.g. a partnership, joint venture, private action, et cetera) within the jurisdiction of the United States, the government could seek an injunction or other legal remedies against your interference and damages arising from your actions on Earth.
Whether this could actually be regarded as a criminal act is another matter; from a quick reading of 14 CFR I can’t see that there is any established law regarding the interference with or theft of objects on celestial bodies. If you brought the rocks back to the United States as an independent organization you might be charged duties for importation (for which NASA is explicitly exempt) but sending them to another destination would seem to be beyond the scope of the Code of Federal Regulations. However, you would probably not want to try to send it directly into the Sun owing to the prohibitive energy cost of doing so; slinging it outward would be much easier and then do a gravity maneuver with Jupiter or Saturn to kill the orbital momentum, or else plotting a complex swingby trajectory to Venus-Earth-Venus-Mercury or something similar to remove the orbital component of velocity.
Getting back to the mutable laws of Earth rather than the inexorable laws of gravitation, there is the hypothetical crime of vandalism or theft of government property, not of the samples themselves, which are arguably not in possession of NASA until they are actually within a vehicle but rather the canisters that NASA delivered and are marked as property, but you could also argue that these are salvage since there is no currently defined mission to actually collect these. Hypothetically, I suppose you could return them to Earth and claim a salvage bounty for them, and were it less than the cost of launching their own mission NASA might even be willing to pay you for it.
The only treaty that really discusses celestial bodies beyond Earth orbit is the “Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies”, a.k.a. The Outer Space Treaty of 1967, and the pertinent articles are VII and VIII, reproduced below:
Article VII
Each State Party to the Treaty that launches or procures the launching of an object into outer space, including the moon and other celestial bodies, and each State Party from whose territory or facility an object is launched, is internationally liable for damage to another State Party to the Treaty or to its natural or juridical persons by such object or its component parts on the Earth, in air space or in outer space, including the moon and other celestial bodies.
Article VIII
A State Party to the Treaty on whose registry an object launched into outer space is carried shall retain jurisdiction and control over such object, and over any personnel thereof, while in outer space or on a celestial body. Ownership of objects launched into outer space, including objects landed or constructed on a celestial body, and of their component parts, is not affected by their presence in outer space or on a celestial body or by their return to the Earth. Such objects or component parts found beyond the limits of the State Party of the Treaty on whose registry they are carried shall be returned to that State Party, which shall, upon request, furnish identifying data prior to their return.
I would not assume my reading of the subject is the last word since ‘space law’ is really a nascent area of jurisprudence, and you would be wise to seek other, more expert opinions. To that end I would suggest that you advance your question to LegalEagle, perhaps with the suggestion that he enjoin Scott Manley and/or Vintage Space curator Amy Shira Teitel, not because I think you’ll get a more definitive answer but just because I would find it entertaining to watch a KSP simulation and hear about the history of ‘space law’.
Stranger
& 
messages in bottles, they’re just randomly dispersed, not intended for anyone specific, which yes, makes them a kind of litter.