The traditional and still most common way to say "to disappoint" in Esperanto is "seniluziigi". Literally this means 'to deprive of illusion'. But already Zamenhof used it in the sense of 'to disappoint'. (It is important to note that the meaning of Esperanto compounds is often not identical to the sum meaning of its parts. The fact that compounds can have an independent life is important for them to be a really useful part of the language.)
Another option is "elrevigi", literally 'to take out from a dream'. (Note that "revo" is only a dream in the sense of a positive idea one has while awake. The dream that you have while sleeping is called "sonĝo".)
Both of these options are more common and more widely understood than "desaponti", and unlike "desaponti", they are composed of official word roots and affixes (official as in either in the Fundamento de Esperanto or in one of the Oficialaj Aldonoj).
Note that the intransitive forms "seniluziiĝi" and "elreviĝi" ('to become disappointed') are actually more common than the transitive forms.
A very recommendable website to look up such official and more common alternatives to unofficial and/or uncommon words is Simplaj Samsignifaj Vortoj (SSV), which is managed by Renato Corsetti, a famous member of the Akademio de Esperanto, in collaboration with other accomplished Esperanto speakers. The recommendations on this website are based on the principles set out by the famous Claude Piron in his book La bona lingvo, a book that eloquently pleaded for the use of compound words when possible, instead of always introducing new word roots. Here are the recommendations that SSV has for "desaponti" and "desapontiĝi":
desapont·i → sen·iluzi·ig·i
desapont·iĝ·i → sen·iluzi·iĝ·i, el·rev·iĝ·i, dis·rev·iĝ·i
The symbol "→" here means that SSV advises against the word before it, and recommends to use one of the words after it instead. Some entries in SSV have the symbol "~" instead, which means that they don't advise against the word in front of it, but still present some possible alternatives to it.