Please if it doesnt come off then do not force it off, you can do more harm than good.
You will need to put a lot of effort into treating them - because if you dont it will just get worse and will unfortunately need to be seen by a vet, and you already made it clear that the vet is not an option.
Heres the treatment, this is from Austins Turtle Page:
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This is commonly referred to, and what most keepers think of, as Shell Rot. Most often, this is due to a scratch or other injury or scutes that are beginning to shed and have trapped water. This trapped water begins to fester with bacteria and/or fungus. A preventative measure in this case would be to provide proper basking spots where the turtle can be allowed to get completely dry.
Listed below is a regimen that I have followed in treating shell problems and has yielded great results. Keep in mind that shell injuries will take months and sometimes years to get back to looking normal if the possibility exists. It takes weeks before the shell will look like it's getting better. While treating, you are not looking for signs of healing or repair - you are looking to ensure that it is not spreading.
Clean infected areas thoroughly with a strong, undiluted betadine, iodine or Nolvasan solution. Let the turtle air dry in a warm setting for about 45 minutes.
Apply a generous coating of Silvadene cream. Work into problem areas.
Leave the turtle dry and warm, ensuring that you do not over-heat, for 18-21 hours each day.
The next day, scrub entire shell with a one of the previously mentioned solutions, using a soft-bristled toothbrush
Place them into fresh, clean water. Let them swim, drink and eat for approximately 1 - 1 ½ hours.
Repeat procedure from Step 1.
P.S. aside from nolvasan and betadine, you can also use hibiclens which you can find at any walgreens or CVS/pharmacy. Adding fresh water aquarium salt to the tank isnt a bad idea either.
