Commenting is allowed for the ideas when they are available for voting. However the commenting is available even after it is completed i.e. Not considered, Duplicate, etc. Once the idea is closed it is not feasible to vote. I think when the voting option is disabled, the comments on that idea may not be getting enough attention? My suggestion is to disable commenting after the idea is completed i.e. Not considered, Duplicate, etc.
Note : I am open to take suggestion and modify the idea if required.