How to cite ACM Computing Classification System?
Since you want to use the ACM Computing Classification System as a data set, rather than a publication, I recommend to use rules, established in the publication style guide for citing data sets. The only potential change I would consider is substituting the type of reference from [Data set] to [Data set -- Ontology] or, even, just [Ontology]. That is, of course, if your institution is not too inflexible on deviations from publication style standards. Otherwise or if you want to be on the safe side, just use the standard [Data set] identifier.

Check the following relevant resources on citing data sets, based on APA as well as other publication style guides. It should not be a problem to convert your ACM CCS citation to BibTeX format by using @misc entry type and including [Data set] into a relevant field (location-wise).



Resources:


Official APA blog post "How to Cite a Data Set in APA Style";
Examples by MSU Libraries How to cite data: Dataset citations (various styles);
Excellent comprehensive article "How to Cite Datasets and Link to Publications".




UPDATE: (initial attempt - alternatively, delete publisher / address and use "ACM" as author)

@misc{Acm:2012,
  author = "",
  title = {{ACM} Computing Classification System {[Data set].}},
  howpublished = {Retrieved from \url{http://dl.acm.org/ft_gateway.cfm?id=2371137&amp;ftid=1290922&amp;dwn=1}},
  note = {Accessed: 2015-06-21},
  year = {2012},
  publisher = {Association for Computing Machinery, Inc.},
  address = {New York, NY}
}