How to remove unused dependencies from composer?

In fact, it is very easy.

composer update

will do all this for you, but it will also update the other packages.

To remove a package without updating the others, specifiy that package in the command, for instance:

composer update monolog/monolog

will remove the monolog/monolog package.

Nevertheless, there may remain some empty folders or files that cannot be removed automatically, and that have to be removed manually.

following commands will do the same perfectly

rm -rf vendor

composer install 

The right way to do this is:

composer remove jenssegers/mongodb --update-with-dependencies

I must admit the flag here is not quite obvious as to what it will do.

Update

composer remove jenssegers/mongodb

As of v1.0.0-beta2 --update-with-dependencies is the default and is no longer required.