OK. Based on andresrcs's suggestion, I put the following line into the .rprofile of the working directory of my "base" project where I do all my package maintenance:
install.packages <- function(pkgs) utils::install.packages(pkgs, INSTALL_opts = '--no-multiarch')
This in essence replaces the native install.packages() with a personal version that includes the "--no-multiarch' option. It works for initial packages installs, and since update.packages ultimately calls install.packages(), it works for update.packages() as well. So I can finally get rid of the 32 bit versions that I never use.