If I understand the problem correctly, you want to basically disable/block the valve on the 3M #8511 type masks that you have access to, correct? The idea is to make it function like the 3M #8510/8210?

If so I suggest two options:
1. You could apply glue using a hot glue gun in/around the valve to keep air from moving through it.
2. You could print a simple cap that goes over the valve and glue it in place for the same effect.