Use a pack of cheap plastic cutlery!
I recently cleared one of my micro-beds for the climbing beans, and as soon as I cleared it, the local cats started scratching around in it. To keep them off, I simply stabbed a handful of plastic cutlery into the ground around the seeds, and it's worked brilliantly.
Not only do I have an amusing modern art installation (particularly funny for the first couple of weeks, before the seeds sprouted), but if I spot a weed while I am out there, all I have to do is lift out a fork, use it to weed or fluff up the surface, then push it back in to do cat-repelling duty. And in a couple of weeks, when the bed is full enough not to be of interest to the cats, I can pull them out, wash them off, and put them away for use again next year.
Memo to self: don't forget to mark them clearly "garden use only" !