OK, here's the thing. You won't get any options with a right click on your user defined parameters if those parameters are using dimension names that you have defined yourself.
So let's say you have a box with a length and width dimension called 'length' and 'width' and you have a parameter that says the height of your box is going to be length x width... You won't be able to delete that user parameter until you go into your relations window and rename 'length' to 'd1' and 'width' to 'd2' or something like that.
Give that a try and let me know.