Drennan hooks are supplied in thick plastic envelopes about 2/3" square. Put used hooks in an empty pack, and slide the tongue of the envelope in the slot, and chuck the envelope in the kitchen waste bin.
For extra security you could sellotape the tongue if you want
I drop them down the drain at the back of my house (one which only I'm ever going to stick my hands down to clean!). I figure that they will rust to nothing in a few weeks.