How do I remove printed letters from coffee sacks?

I think they are on there permanently but you could probably make a patch and attach the patch over the letters.

The letters are permanently dyed in, most likely. The brand is put on there so you'll remember it.

You might try bleaching which will lighten everything and probably weaken the fabric.

Can you use the other side of the fabric? Is it clear of ink? I would think the texture would be similar.

Could you turn them inside out so the lettering is on the inside?

I would turn them inside out , then if you want to hide the seams , either re stitch around the bag of cover them with ribbon or trim .

What are the coffee sacks made of? Are they plastic? If so, perhaps nail polish remover could be used to remove the letters. If the sacks are made out of burlap, you may need to cover the letters with a patch.

How about turn inside out or paint then or cover them with bigger stencils

Might try Rit's Color Remover where ever they sell Rit Dye.