All you can do is keep cutting the top off.If its a tree this will not do much for its shape but its the only way. When buying a tree its always wise to check what its mature height will be.

Creative pruning to form a pointed top is possible, if not too much height needs to be removed. The secret is to prune the leader and other stems back to green, upright side branches, instead of hacking back with hedge shears. The resulting thin spots will fill in, especially with judicious organic feeding, including a little Epsom salts. The best time to do it is in early spring.

Yes and you can keep it tidy after pruning by tying it round with a pair of old tights, obviously taking it off when the tree has got its shape back. (First remove the wearer!)

Camomile,do you think that would work with a fastigiate berberis that's getting rather bushy?

To be honest Stera I'm not sure but they do seem to have quite pliant branches judging by pictures I've seen, so nothing ventured ........ It can't hurt it.