If something is spoken for, it is not available because someone has already bought or asked for it.

Most of the best paintings in the exhibition were already spoken for. 展览中大多数最好的画作都已经有人订购了。

If someone is spoken for, they are not available for a romantic relationship because they are already having one with someone else.

Both girls were spoken for. 两个女孩都已经名花有主了。