Does pension plan = 401k in this case? If so, Enron didn't prevent anything. Regulations regarding 401k plan administrators prevented them. Good old governmental regulations to "protect" participants once again came around and bit them in the rear.
As for company stock in general, our company once had a dollar-for-dollar match. I could buy 10K in company stock and they would match it but you couldn't sell the matching shares for 5 years and could only get the match once a year. That's to prevent me from buying 10K in stock, getting the matching shares, selling them later in the day and repeating. They later went to a 10% discount on stock purchased by employees provided you keep it for two years. I could pay 9K for 10K worth of stock, but they were encumbered shares and couldn't be sold for 24 months. You want shares you can sell at any time, don't go for the discount.
There may have been restrictions on stock sales for reasons like those, but Enron did nothing sinister from what I've heard - At least not regarding employees and company stock.
They were changing the 401K is some manner such that all of the stock was locked up for two months or so. This of course was advertised way in advance. Enron belongs on the list for being liars and frauds, which cost all stockholders, but not directly for idiots who put all of their eggs in one basket.