When you forgot to use secrets from the very beginning - and some secrets landed in your repository eg. login/password/secret_key you can remove them in a simple way using filter-branch for example to remove password you need to use:

git filter-branch --tree-filter "find . -type f -exec sed -i -e 's/password/**REMOVED**/g' {} \;"

It will replace password with **REMOVED** in the whole repo and commits.