Sorry... this is going to be a bit of a rant.
If you are in a gym and train around others, you should learn to spot correctly!
If the person requesting a spot tells you not to touch the bar unless he calls for it. DO NOT TOUCH THE BAR UNTIL HE CALLS FOR IT OR IS FAILING.
I had my bench workout today ruined because some trigger happy, spotting-newb would not keep his hands off the bar while i was benching. Completely ruined my rep cadence and tension.
Furthermore, while spotting for the bench press, its usually best to use a mixed grip. If the person under the bar completely fails, the mixed grip will allow you to hold on to the most amount of weight. Plus it allows for a stronger grip.