From The Secret - No-one else can bring negativity into our lives through their thoughts, unless we allow our frequency to lower to the same negative frequency as theirs.

Anybody who thinks negative thoughts about someone else has those negative thoughts return to them multiplied. It doesn't matter how many people are thinking negatively about someone; if that person is in joy those thoughts can never reach them, because they are on a different frequency. Instead, all the negative thoughts will multiply and return to the people who were thinking them.
No-one else can bring negativity into our lives through their thoughts, unless we allow our frequency to lower to the same negative frequency as theirs.