Answer

In principle, they also prohibited it even if it was not made for idol worship, but that is only if there is a complete human form there, with all its limbs.
“Some say that they prohibited a human form or a dragon only when it is a complete form in all its limbs; but the form of a head, or a body without a head, has no prohibition whatsoever, whether one finds it or makes it” (Yoreh De’ah 141:7), and the Rema adds that this is the accepted practice.
However, when it was not made for idol worship, this is a rabbinic prohibition, lest people think it is for idol worship. Therefore, several halakhic decisors wrote that when it is clear that it was not made for idol worship but rather for play or the like, one may be lenient. Here we are dealing with an ashtray, which was certainly made for a specific use, and therefore in my opinion one may be lenient even if it contains a complete human form.