Objective values can be defined as qualities or entities that are objectively good for and add to the well-being, guiltless pleasures(the only kind), and long-range happiness of the human organism.
There is no way you can prove that any such thing exists. You can claim that it is subjectively good for you; but that's the exact opposite of an "objective good."
Absolute objective values to all human beings can include such basic and unchanging values as productivity, honesty, justice(not mercy- what we have now), self-esteem, pleasures, sex, and romantic love.
None of those are objective values; in fact, rendering them into "values" is one flaw of the human condition (and a form of mysticism).
Take "honesty" for example; the only reason you claim it as a value is because you're thinking within an epistemological framework with strict parameters on life and language. You think you're being less mystical, but you're actually more mystical.
Say hypothetically that aliens arrive from a distant galaxy, and they communicate through a telepathic/empathic medium. For them, there is no spoken language, entities merely experience and understand each other through an emotional medium. For beings such as this, "honesty" as a value doesn't exist. They can't differentiate between "true" and "false." For them, there is no "lying;" and thus, there is no truth. A situation such as this shatters the illusion that honesty is a value; it's not an objectively good thing, it's merely a binary we've constructed since we work within a specific set of epistemological parameters (and we feel the need to narrativize our existence).
Now, a potential rebuttal might be that "well, humans can't read each other's minds, so for us honesty is a value." But simply by saying that, we've undercut the idea that it can be an "objective" value. If it is only valuable "for us," that means it's not objective. And if it's not objective, then it isn't necessarily good for all humans either (in fact, there are plenty of situations when lying is good).
Surely, you have to acknowledge that some things are objectively good for mankind and some are objectively bad.
You just proved my point right there.
There's a great book on how all these things break down; it's called
The Shape of the Signifier, by Walter Benn Michaels.