As already stated, just hit "Enter" on an empty input parameter and it is deleted, it will then present you with an "empty" return parameter (in a JSR), but if you don't want that either just hit "enter" again.
This is just RSLogix doing its best to make your life easier.
If you are just entering a JSR without parameters, then yes I agree it does seem to be a bit tedious, but when you do need to add a series of parameters to a JSR it saves a lot of clicking here, clicking there, right-clicking and left-clicking, "add input parameter", "add return parameter", etc., etc.
Once you get used to it.........