I have someone like this too, who has a basic knowledge of some basic magic effects and sleights. Every time he sees me perform, one of two things happens.
a) I turn over a card and he shouts out "DOUBLE FLIP!" regardless of whether it actually was a DL or not.
b) After a card is selected and I use some sort of mixing procedure to shuffle the deck, he always shouts out that the card is back on top.
OR, both a and b combined. He calls out that I have controlled the card to the top. I show him otherwaise and he shouts out "DOUBLE FLIP!"
Get one really good fooler, something he would never be able to figure out the method of.
One thing you can do is... if you see him coming before you start performing something is..say.." oh wait, I know that so and so is a great magician also... everyone give him a hand... " then Give him your cards and say..' you're on kid.."
I doubt very highly that he has any tricks he can perform... after all if he could he would not be telling people how YOU perform.
Then the next time you see him...Just say..." Wait... he's back, ready to show us a good magic effect???" " No???"
Another thing is... work on making your magic theatre that happens to use cards as your vehicle... make a LOT of eye contact... connect. This way they will see your performance as such, and they will see that loser as an interruption.
Another thing is...Once you have your audience, whenever he comes by, if the " you ready to perform for us today?" remark doesn't chase him off is say.." I would LOVE to perform for you, but I cannot do so while Hjo schmo over there is determined to shoot me down..." They will chase him off to see you perform.
One last point. Are you in school? if so, I think that is the worst place to perform magic. A lot of kids feel a competative urge to take away your attention. It's what the School system does to kids. And they will try to take attention from you whether positively or negatively. Not the best environment for magic.
PS: get Heckler by Brian Tudor
. This is the exact scenario this routine is for, since it plays on the heckler believing he knows where the card is, and you basically always having it somewhere else. You basically have him come foreward, and sign the card... and say.." Ok, try and keep track..i know you'll fail..but this should be fun" then smile at your audience. Now... after 3 or 4 times that the card is Not on top..but ..in your right Pocket...left pocket...jacket, pocket... shirt pocket... etc... if you are good at these sleights he'll never guess right, he'll always look like a schmuck... all to the enetertainment of the audience... trust me he'll avoid you like the plague... do NOT feel sorry for him, he is trying to gain some popularity at your expense...cut him off at the knees...