The analogy is invalid. Spam is something nobody wants to receive, by definition. The people on Parler voluntarily wanted to communicate with each other but were prevented from doing so by a third party.
A formulation of freedom of speech that might be useful to contemplate is Article 19 of the UN Universal Declaration of Human Rights:
"Everyone has the right to freedom of opinion and expression; this right includes freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers." Deplatforming by a third-party that isn't part of the conversation definitely runs afoul of the "seek (and) receive" part of that ideal.
I don't mean this question in bad faith, and I support spam blocking, but spam blocking seems incompatible with a free-speech absolutist position.