Actually themselves is correct, the word themself (hint: there is no themselve) is the less formal version that aims at being a singular reflexive pronoun, but isn't widely accepted at this time. The alternative would be himself / herself, but this requires to work with the gender which isn't always known and mixing terms can be confusing.
USA = This post was deleted by the author themself.
USA = This post was deleted by the author them self.
USA or International = This post was deleted by the author themselves
Oxford English Dictionary Online does not have an entry for themself. However, under the entry for themselves it states that "themself" was once the normal form of the third person plural reflexive pronoun until it fell out of favor and was superseded with themselves, which became the standard form.
Although some current dictionaries, for example, The New Oxford Dictionary of English, state that themself has re-emerged in recent years when used to refer to a singular gender-neutral noun or pronoun ("themselves" remains the normal third person plural reflexive form), they label it as "rare" or "disputed" or "not widely accepted in standard English".
Other dictionaries such as Webster's New International Dictionary do not mention it at all. Yet Webster's American Dictionary does list both "themself" and "themselves".
In sort, this really is dependent on geographical dialect.
(Though it is not a bug since WBB needs generalized international English as it is sold and used, the world over. Just thought I'd comment for comment sake)