Not very much. Planting them deeper would have a marginal effect - on the height anyway. You can't cut them back.

What do you have? Longiflorum-Asiatic hybrids? Do they fall over or are they in a spot where the dying back stem can easily be seen?

I'm really curious as to what asiatic gets to 6 ft.?

There are the LA's (longiflorum-Asiatic hybrids) that can. But if this is another in the long series of hit & run posting that occurs on so many of the forums now - we'll never know.

I know of several Asiatic lily hybrids that can approach 6 feet tall. Many of them are the hybrids with pendent, down facing blooms. The Lily Garden catalogue lists hybrids like 'Karen North' as growing 6 feet tall or taller once established.

Planted in Fall 2011, my first year stems of 'Karen North' are barely 24 inches tall. However, my first year stems of 'Citronella' planted at the same time are approaching 5 feet tall!

As for the original poster's question, I don't think it is possible to control lily height.

You can keep a lily a little shorter by digging and replanting each fall. Also, stop any suplemental watering and feeding. Lower bud count is a negative consequence of stunting, however. A trick I use in my mixed lily show garden is if one or two of the permanant resident lilies gets a little 'out of hand' is to do a partial 'lifting' in the fall( putting the garden fork under the plant--prying up a couple inches--then setting back down. This has only a limited affect on height.

It is as ISPAHAAN says: There are Asiatic hybrids out there that get 6 ft or more and its technically not possible to control a lilies height significantly. There are a few Division 8 hybrids, especially within the Oriental Trumpet group that approach 8 ft. Where will it end?