What is the opposite form of by adding a prefix?

There isn't a direct opposite of "by" formed by adding a prefix. "By" is a preposition indicating means or agency. Its opposite would depend on the context. Here are some examples:

Instead of "by" indicating means:

* Without: "The task was completed without any help."

* Through: "The work was done through hard work."

Instead of "by" indicating agency:

* For: "The decision was made for the good of the company." (Opposite of "by" as in "made by the company")

* Against: "The proposal was voted against by the board." (Opposite of "by" as in "voted by the board")

To get a more specific answer, you need to provide more context. Tell me what "by" is referring to in your situation.