I have never had this problem but I suspect that its root system is huge by now and provides it with quite a bit of energy to support this behaviour. Perhaps you could use a spade to cut the roots by inserting the spade around the shrub, maybe 2 to 3 feet away from the main shrub and then pushing it vertically downwards. Basically, pretend you are going to transplant it but just insert the spade 6" to one foot deep at most.

It would be theoretically possible to use a plant growth regulator to keep the new growth shorter, thus helping to prevent the sprawl and flop. Forced potted hydrangeas are treated in this way. However, it would take a lot of trial and error, and the product used, called B-Nine WSG is very expensive and would be hard for a non-professional to obtain. Vendors do sell to the public on E-bay at times.

A better option might be to use some kind of a horizontal grid type of support, similar to the ones shown in the link. You could probably make something along those lines using wire fencing and wooden stakes that would be better than the commercial ones.

Here is a link that might be useful: Google shopping results for plant support rings

I have the same problem with an overly huge plant. I have a lacecap that is probably 12 feet wide and 8 feet tall. It's eating the front of my house. I've never severely pruned it, but this year I think I'm going to cut it way back--probably with a chain saw. ;-) I'm afraid that it will just grow back to the same size.

The irony is while I love mop heads, I am really not that excited by lacecaps. It was here when we moved in.