How do you dull a knife blade without special tools?

Well let’s use the opposite factor equation here.

1. It takes really fine grains of rough edges to sharpen metals. (For instance the upside down of a mug, sand paper, sand etc)

Using that logic we can think of opposite things that aren’t rough. Hitting wood board whilst cutting meat has done the job for my knives over the years. So go ahead and hit some wood. Also you can hit metal that is of a higher grade than your knife, say like a hammer.
Just gently tap the knife with the hammer head, evenly distributing the hits.

Run the blade lightly over some sandpaper. Staple or nail the sandpaper to a wooden block so you don’t accidently cut your hand while holding the sandpaper.

A fine grit paper should give you a more even edge than coarse grit.

A minute or two with a bench grinder should do it.

Go out into the yard and repeatedly stab the ground or saw down small shrubs, if you’d like to come over here you can use it to prune back my giant bamboos, that oughta do the trick.

ok, thanks a lot, everyone! Although I think I’m getting conflicting advice from @auhsojsa and @WestRiverrat . I previously read somewhere that sandpaper can dull a blade, so I’ll try that. Thanks again!

@basheersubei it depends on how you hold the blade, the same medium used to sharpen a blade is usually also the best medium to dull the blade. If you have a whetstone, you can dull the blade by running the edge directly over the stone at a 90 degree angle, instead of the usual 12–20 degree angle that will sharpen the blade.

Coaser material will dull the blade faster but is more likely to leave nicks in the blade, the finer material will give you more control if you don’t want to damage the finish on the blade.

Yes what @WestRiverrat says ! the form of angle you hold the blade controls alot

Ok, guys! sandpaper and wood-hacking worked. Thank you!