For some kind of objective evidence of what I would otherwise say was a matter of opinion, you can look at Google Ngram Viewer, which tracks the number of times something appears in print.
Per this resource, spoil your appetite is the most common. This is followed by ruin your appetite, which gets about a third of the use, and decrease your appetite trails with about a sixth of the use.
Changing the corpus to look only at US and UK English shows the same commonality, except that decrease your appetite seems to appear only in printed form in the US—it has no measurable use in the UK.
Interestingly, if I query the frequency of just spoil versus ruin, I find that ruin is the more commonly used verb outside of any particular context.
Asking which is more polite is something that cannot be captured objectively, as far as I know. Trying to answer that would seem to be entirely subjective or a matter of conjecture.