Cutting blooms from peace lily won't kill it, it will grow better in my expirience, since blooms are actually consuming energy off the plant. By removing it, the plant will grow more leaves and new blooms when the condition is right.
Since the petal-like white thingy (spathe) is actually a modified leaves, not a true flower petal, I believed it could be dried just like most leaves, but not sure of how it would look like, it could be wrinkled, or changed color bit.
I never tried it on Spathyphyllum though, but would really want to see your result.
May be a dessicant such as silica dessicant would work for this, but it would take some times, since those spathe is thicker than most flower petal.
And a clear acryllic spray might be usefull as the final coating.
If you decided to do it, I wish you can post your result here..
wish you the best on this project
