I discovered that using a placement file like this
<Match Path="/recrutement/*">

    <Place Parts_Title="-" />
</Match>

removes all the titles for the pages under /recrutement/* but not for the pages built with a custom form.
Their Title are still rendered with an h1.

So there is no way to remove the title of a custom form easilly.

<Match ContentType="YourContentTypeUsedInCustomForm">
  <Place Parts_Title="-" />
 </Match>