None of the DRAE definitions of "decantar" seems to fit the bill here, but I think the meaning is related to "decant":

"Separar un líquido del poso que contiene"
http://lema.rae.es/drae/?val=decantar

However, it is a metaphorical meaning derived from this. When you "decant" something, by this definition, separating the liquid from the dregs, you purify it. "Decantar" is used in this sense in alchemy, for purifying matter:

"símbolos de la alquimia que facilitan la manera espiritual de decantar y de transfundir la materia"
http://www.cervantesvirtual.com/obra-visor/sur--2/html/027eb...

One of the dictionary definitions of "decantar" in Portuguese is precisely purify:

"de·can·tar 1 - Conjugar
(francês décanter)
verbo transitivo
1. Proceder à decantação, à filtragem de.
2. Transvasar um líquido para o separar das suas impurezas.
3. [Figurado] Limpar, purificar.
"decantar", in Dicionário Priberam da Língua Portuguesa [em linha], 2008-2013,
http://www.priberam.pt/dlpo/decantar

And "décanter" in French can mean clarify.

So I think "decantar" is being used in this sense: metaphorically remove the impurities, and so purify:

"Queremos suprimir lo superfluo: decantar."
http://elistas.net/lista/redluz/archivo/indice/1760/msg/1631...

I've found other examples of this usage in relation to Toltec thought and ancient Mexican philosophy generally:

"Aquellos seres humanos que estaban dispuestos a hacer un sacrificio excepcional y un esfuerzo superior para decantar su materia y purificar su espíritu a través de La Batalla Florida, que se libra en lo más profundo de su corazón [...]
Xipe Totec es el simbolismo del acto consciente de decantar la materia. Es decir, desprenderse de aquellas pasiones y debilidades asociadas a la “carne” y al mundo material [...]
De la misma manera simbólica, la materia muere pero resucita con mayor fuerza con el espíritu decantado y purificado"
http://toltecayotl.org/tolteca/index.php?option=com_content&...

The second passage here indicates that purging or freeing oneself of materialism, as Jenni and James have suggested, is very close to the meaning. But "la materia" is matter, the material world, the material self, not materialism. Ridding oneself of material passions and weaknesses is an explanation of what is involved in "decantar la materia", but it is not actually what the phrase means. It means purifying matter, and matter refers to one's material self (as opposed to "espíritu", one's spirit or spiritual self).