Various explanations of the origin of the SPAM name include "SPiced hAM" and "Spiced Pork And haM"; the current official expansion is "Specially Processed Assorted Meat" as the SPAM Lite variety contains both pork and chicken meat. According to [Hormel's trademark guidelines], you should spell SPAM with all capital letters and treat the mark as an adjective, following it with a more generic descriptor, for example "SPAM luncheon meat".
Official information from Hormel Foods about the SPAM trademark can be found on [this page].
See also spamming.