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What you are telling grep is:

• Print only foo for each line that has it, and
• Print lines that don't have a foo

What you meant to tell grep was:

• Print lines that don't match foo, or the parts of lines that don't include a foo

That can be reduced to:

• Print every line, but remove foo from them

I think sed is the perfect tool to do this:

$ sed 's/foo//g' ./file bar bar 

This says: for each line, substitute foo with nothing globally. By globally it means every instance in the line.

If you also want to delete empty lines:

$ sed -e 's/foo//g' -e '/^$/d' ./file bar bar 

This says:

• substitute foo with nothing globally, then
• for each empty line, delete it