Is This Day Over Yet????????

Okay, everyone out there in blog-land. I have an issue that I totally need suggestions and advice for. In my fourteen years of parenting, I've never had this issue- at least not long term. Here it is...
My two oldest daughters, Alexandria and Sydney are causing problems at naptime/bedtime. In the form of
1)Alexandria takes off her clothes and/or wets the bed at every single naptime and bedtime
2)Sydney takes off her clothes and diaper and then potties (#1 and sadly sometimes #2) in her bed every single naptime.

Every afternoon/morning I find them naked with messed up sheets. This goes on twice a day. Every single day. Laundry is piling up because two loads a day are taken up with the girls bedsheets and blankets.
Here are the things I've tried already

-talking about it with the girls. They seem receptive. They tell me they understand. They nod their heads in agreement when we say "no more taking our clothes off in bed. No more wetting or pooping the bed." Then, hours later, they become repeat offenders.

- Spanking. Yes, I've resorted to spanking when I became sure they knew what they were doing. And yes, they know. They hold their bottoms and yell "no, no, no, I don't want a spanking!" when I come get them up from their naps or in the morning. They seem upset at the moment of spanking and seem to understand when we discuss the issue afterwards. But hours later, they become repeat offenders.

- shortening their naptime and waking them up earlier. My thinking was that maybe they are simply in there too long and get bored. Shortening naps, and getting up earlier in the morning have not helped.

-making sure Alexandria goes potty RIGHT before bedtime and naptime. Making sure Sydney has a BRAND NEW diaper right before nap/bedtime. Really, these are things I did anyhow.

So, dear readers...what should I do???? I'm going to scream if I have to launder another sheet/pillowcase/blanket/bedspread.
Advice please???????