Well, I hope you like the ship you are flying (and never get blown up) because if the years hardly pass the shipyards will hardly update as well. Slowing things down is an option, but a simple way to get rid of the X-Ships is to place an "R" in the Special Role section of the shiplist for each X-Ship (open the shiplist with a spread sheet editor like MS Excel to do this, replacing any other letter in the slot, if any, and back up the file before making any changes). If you are using the stock shiplist there are only a handful of X1 and X2 ships to edit. If you are using OP+, then there will be more. This will make it so they never appear anywhere in the campaign for you.