Could you please give an example of a word where th is hispanized as z?
By the way, the spanish phoneme 'z' is pronounced differently to the english phoneme 'z'.
We can do that in "think" or "thanks". But not in "they". In the latter case we tend to mispronounce it as a Spanish phoneme 'd'.
Taken from Wikipedia https://en.wikipedia.org/wiki/Spanish_phonology#Consonants
"The phonemes /b/, /d/, and /ɡ/ are realized as approximants (namely [β̞, ð̞, ɣ˕], hereafter represented without the downtacks) or fricatives[6] in all places except after a pause, after a nasal consonant, or—in the case of /d/—after a lateral consonant; in such contexts they are realized as voiced stops.[7] (In one region of Spain, the area around Madrid, word-final /d/ is sometimes pronounced [θ] especially in a colloquial pronunciation of its name, Madriz ([maˈðɾiθ]).[8]) "
And, if the case of participles, we just nearly butcher the in-between 'd' in -ado as -ao, simillarly to the Southern speakers from the US on lots of words.
What I meant is that the closest sound to "th" in Spanish would be "z" or "d" instead of "t", as other replies have pointed out.