You may not like my answer, but here is what I did on my Prisma 870. I took it apart, many YouTube videos to help with this. You may not want to go this deep.
Then I removed the upper ink absorption pads from the right side base of the machine. These I rinsed in warm water until clean. Then rinsed the lower pads, which I left intact ( too hard to remove), squirting water on them then sucking it out with a wet & dry vacuum, I did this about 10 times. It is incredible how much ink is in the bottom of these machines. Then I reassembled everything.
Next I downloaded the canon utility ST_V3600_resetters (canon site). I hooked my printer to my computer by cable. Then put the printer in service mode. You should easily find an instruction on YouTube on how to put your model in service mode. My computer recognized the printer right away. I then ran the utility program. The utility didn't work 100%. There is an option to set the region, which didn't work for me, so a lot of the text was in Japanese, but not all of it. The block for Ink Absorber Counter and Counter Value was in English. My counter was at the max (I think it was 1000), I set it to zero and hit the set button. Bam, my counter went to zero and the printer was 100% as new. I have been using it the last two months and it is perfect.
I wish I kept a more detailed step by step, but honestly I thought I had little chance of success. I was wrong.
Do a search on ST_V3600_resetters, it easily comes up on the net.
Good Luck!
Rick