If the person has affected you, I would say it's because you've let your shield down. While some believe you can't be affected, and that you're convincing yourself, I say it's half true. Energy is energy, and how you allow it to affect is up to you. Just as you can send/receive someone positive energy, you can do the same with negative energy. However, you can combat that with your shield. That's why you'll hear so many say over and over "Use a shield!"  The rainbow bubble you'll hear about is nice, but through some digging I've found using various lights and elements for a layered shield works great. Pick what you want, but I've found layering my shield works great. In the end, what you visualize is what helps best. We're individuals, after all.

I have 2 forms of 'shielding' that I recommend/try.

1.  Visualize yourself wrapped in a cocoon of pink light of love & protection.

2.  Visualize yourself in a bright & brillant tin can.  Safely inside, reflecting all negative energy back to the person, while reflecting also your own beautiful light of truth & love

IME NEVER use mirrors of any kind in your protective shield.  What you mirror back is your karmic responsibiity.  What they sent out is theirs.  

Mirrors are just not the best tool in any way in psychic work.

Lol, I tend to get the willies around mirrors at night. Maybe that's why, considering I'm a dreamer.

girlsmomma, what I use is similar to a combination of both. I wrap myself in colored light, starting with white, pink, then I go through the spectrum depending on what I feel needs the most. I use woods and metals, typically ending with silver and gold. In between I put a "sticky substance". That fills in any cracks. I guess you can say I'm covered.

I have never heard of powerful mean psychics yet, but what you can do:

1. Use the Law of Attraction to your advantage, do not think about stuff like this and mean psychics won't exist in your life
2. Use a bubble shield. Like a dense psi ball, you just use it to shield you from any leeching, mind control, empathy etc etc. Note: also useful against mosquitoes since they can percieve energy too XD, but I digress.

Hope this helps