The other option is to add the dimension that you want to the sketch. This will over constrain the sketch, at which point ProE will pop up an info box and the collinear relation that you want to delete will be there for you to choose (click on each line of the info box and it will highlight the affected item on screen to help you choose the correct relation)
Kevin
“Insanity in individuals is something rare, but in groups, parties, nations and epochs it is the rule” Nietzsche