Slander

What can be done about person "A" who constantly tells lies about person "B" to person "B"s significant other in order to disrupt their relationship?

Basically, as I see it, you have the following options:

1) Assume that the relationship that "B" has with his 'significant other' is strong enough for her to see through "A"'s lies, or
2) Move away so that "A" has a hard time talking to "B"'s significant other, or
3) Have "B" cut the phone lines at "A"'s house and residence so that "A" cannot call "B"'s significant other, or
4) Have "B" slash the tires of "A"'s car so that "A" cannot travel to "B"'s and talk with "B"'s significant other, or
5) Have "B" break "A"'a legs so that he can't walk or crawl to "B"'s and talk with "B"'s significant other, or
6) Have "B" beat the hell out of "A" and say, "There is more where that came from!", or
7) Any combination of the above.

Please let me know if I have missed any....

Very funny, so what you're saying is that there is no LEGAL options for "B". Correct?

Correct.

Unless person "A" does a criminal act, or in some way damages person "B", there is no case.

The old adage of "Sticks and stones" seems to come to mind.

With the use of private investigators and email hacking, person "A" has done a good job of fabricating "evidence" of wrongdoings by person "B", all of which has been concocted from out of state!
Some of your "illegal" options would seem plausible, but anything short of killing person "A" wouldn't be good enough! Besides, person "B" couldn't live with that on his/her conscience! LOL! Looks like the "B"s will just have to put up with this jackass.