I know bacteria have these enzymes, but couldnt find in my book on biology (becker and kleinsmith - The world of the cell) anything about it, wo do have these as well, or something like it, right?

Are you talking about reverse transcriptase? In the human body there called telomerases. Yes, we have them associated with the DNA of our reproductive system.

Or are you talking about spliceosome? I'm not sure what your talking about specifically, but yes we have a mechanism by which foreign DNA, say from a virus is destroyed. Its actually beleive that those enzymes are why we need the extra group that differentiates thymine from Uracil. It prevents splicing of our own DNA by spliceosomes.

This is what i have in mind: http://en.wikipedia.org/wiki/Restriction_modification_system
But in humans.

Why i am asking is because some people claim that supplementing with DNA/RNA can help improve cognition and memory, but i find that absurd since everything we eat contains DNA/RNA already, so what im looking for is a mechanism in the body that recognizes foreign DNA/RNA and destroys to be able to show them hard evidence for my claim.

Hmm,
I don't think there is. As multicellular organisms, were adapted to conserve energy over conserve individual cells. Probably what it comes down to, is that its less expensive for an immune response (specifically T lymphocytes that identify and destroy virus infected cells.) to kill a cell, than to express proteins that destroy viruses. If they do exist, they are probably somehow related to the spliceosome enzymes.