Is it possible to not return any data when using a GraphQL mutation?

According to this Github issue you cannot return nothing.

You can define a return type which is nullable e.g.

type Mutation {
  addElement(element: ElementData): ID
  removeElement(id: ID): Boolean
}

But I suggest you return the id of the deleted element, because if you want to work with a cached store you have to update the store when the delete mutation has ran successfully.