[zork]

Couldn't you just disable showParty, showSolo and showPlayer aswell?
That should remove the party display when you are in a raid that meets the requirements.

local raidToParty = CreateFrame("Frame")
  raidToParty:RegisterEvent("PLAYER_ENTERING_WORLD")
  raidToParty:RegisterEvent("GROUP_ROSTER_UPDATE")
  raidToParty:SetScript("OnEvent", function()
    if GetNumGroupMembers() > 5 then
      party:SetAttribute("showSolo", false)
      party:SetAttribute("showPlayer", false)
      party:SetAttribute("showParty", false)
      party:SetAttribute("showRaid", false)
      raid:SetAttribute("showSolo", false)
      raid:SetAttribute("showPlayer", false)
      raid:SetAttribute("showParty", false)
      raid:SetAttribute("showRaid", true)
    else
      party:SetAttribute("showSolo", false) -- show player when solo
      party:SetAttribute("showPlayer", true) --show player unit in 5man group 
      party:SetAttribute("showParty", true)
      party:SetAttribute("showRaid", true)
      raid:SetAttribute("showSolo", false)
      raid:SetAttribute("showPlayer", false)
      raid:SetAttribute("showParty", false)
      raid:SetAttribute("showRaid", false)
    end
  end)

The macro conditions have its boundaries.