The answer isn't fooling them worse...
Here is the deal - does anyone really like to be fooled? Short answer - NOPE!
Did you like when someone made fun of you, made you feel dumb, pointed out something you missed that was really a simple mistake, pick on you for messing up you're...I mean your words? Probably not - unless humiliation is a turn on for you.
More direct - have you ever been in a situation where someone or a group, knows something you don't...they are all on the inside, and you are not? Awkward right?
This is the sub-text to magic - people know you don't have real magic powers - what they also know that creates this, is you know a secret they don't. "If you are so magical, why can't I take the top card" - have you heard this? How about something along those lines?
Here is the solution - make magic fun. Every time I hear things like..."My girlfriend hates magic"...I think...yeah, when YOU do it OR "My friends always try to ruin my tricks"...yep...when YOU do it. It isn't your friends, or the strength of your effects that is the problem - I would bet you are making your magic a painful experiencen for them.
If you are doing magic right - not just with your hands - but with your presentation and personality - the magic should be a vehicle for those things. If you set the mood right, your spectators should WANT to experience what you are doing, they should want to "suspend their disbelief" because they can't wait for the surprise. Your friends challenge you because they feel it is YOU versus THEM, because they know you have a secret they don't and they hate feeling dumb. I think it was Jami Ian Swiss that made reference to magic being a "bitter pill" we force them to swallow when we fool people with no message...other than...I GOT YA...and I would add the word BIOTCH to that.
So - if you want your friends to stop ruining your magic...well, stop ruining it for them. Give your magic meaning - give your effects reason and make it fun...a good test - is it fun even without the deck in your hand?
Some books to read to help you - Strong Magic by Ortiz, as well as his book Designing Miracles. Transformations by Lawerence Hass - really anything that focuses on showmanship, as I am sure you have enough of those. There are many older classic books to add to this list, but perhaps those above are a good start and easy to get your hands on.
In short, make the audience love the magic so much that they feel bad when they ruin it...because it doesn't ruin it for you...it ruins it for them. It is a gift you are giving them, so don't be so selfish about giving the gift. It is not about you - good magic is about those you force to sit through and watch.
Feel free to PM me if you want any advice - cheers