We have two small dogs.
Screens worked for our dogs and cat, but we’ve never had a ferret, so I have no idea if it would deter one. We ended up using a combination of screens and chicken wire to defeat them, as thin screen did nothing. The chicken wire worked best, because it had the “gotcha” quality of being non-obvious but annoying to our resident digger, so you might want the larger, heavier screen that people use for rabbit cages and such. The thin stuff is too comfortable to paws.
The key for us was getting the stiffest wire we could make into the right shape, so it’s uncomfortable to try to dig through.
One of ours loved them, the other couldn’t figure out how to get it to work.
Another possibility is wooden skewers. Cut/break them to a convenient length (a little less than the height of the pot) and stick them down in the soil pointy-end up.
It takes quite a few, you want to have them fairly close together, but it will deter cats from jumping into the planters.
In addition to any of the dissuading methods above, give your ferrets their own soil to dig in. Ours (at the zoo) go mad when they get piles of dirt to dig in.
Another option is a heavy coating of either cayenne pepper or finely ground black pepper over the surface of the soil. Has to be re-applied after any heavy watering, but it works pretty well.
Duct Tape?
Damned if I know have many times to wrap the little bastard though…
A lot.
Over Saran wrap.