How do I block some dates?

Hi,

Very likely, I have posted in wrong category. If so, I'm sorry.

I build a cart a couple of years ago http://stella-polaris.nu/cart/ presently running OC 2.2.0.0. It's a booking thingy I think I made myself without using any extension (sorry, can't remember...).

The flaw with it is that people sometimes don't check the availability in the calendar at the bottom of the front page. They book and pay anyway which causes problems both for themselves and for the owner of the ship.

When they book, they're required to add a date ("Datum"). But the calendar that pops up doesn't reflect which dates are open for bookings or not.

My question: How can I, as easy as possible, block dates (e.g. make them red and not able to select)? Thing is that the owner himself should be able to handle this with as few clicks as possible..

All suggestions are more than welcome!

Cheers,
Dan