The answer to this is going to depend on a few different factors…
- What is the purpose of the spell?
- What are the ingredients?
- What are you current options?
If the spell is meant to banish something or otherwise push something away from you, you will want to dispose of the spell remainders in a place far away from your home. This can include disposing of it in a dumpster that is then going to the landfill, but I would just make sure that the remainders wouldn’t sit in your own garbage can for too long.
If the spell is meant to bring something to you - or any other reason than the one above - it should be acceptable to dispose of the remainders in your garbage can.
I also ask what the ingredients are because some spell remainders can be given back to Earth depending on what they are. For example, ashes can be blown on the wind, most papers can be buried in the Earth, and food can often be left out as long as it doesn’t pose a threat to the local wildlife. Just remember, never bury something or leave something out if it will not degrade/decompose or if it will threaten the local animals and plants.
The last question I ask because sometimes you don’t have many options and you simply have to go with what’s available to you. If the remainder is safe to flush down the toilet or sink, that may be an option. Again, blowing ash to the wind is also a safe bet so long as the ashes are 100% cooled off - let’s not start fires accidentally!
Remember to take into account all factors of your spellwork and then dispose of it in a way that is fitting to the nature of the spell - but also in a way that works for what you have available to you.