Zalgo is largely the result of abusing combining modifiers. Declare that any string with more than n combining modifiers in a row is invalid.
n=1 is probably a reasonable falsehood to believe about names until someone points out that language X regularly has multiple combining modifiers in a row, at which point you can bump up N to somewhere around the maximum number of combining modifiers language X is likely to have, add a special case to say "this is probably language X so we don't look for Zalgos", or just give up and put some Zalgo in your test corpus, start looking for places where it breaks things, and fix whatever breaks in a way that isn't funny.
n=1 is probably a reasonable falsehood to believe about names until someone points out that language X regularly has multiple combining modifiers in a row, at which point you can bump up N to somewhere around the maximum number of combining modifiers language X is likely to have, add a special case to say "this is probably language X so we don't look for Zalgos", or just give up and put some Zalgo in your test corpus, start looking for places where it breaks things, and fix whatever breaks in a way that isn't funny.