How can I make a function work only once or just destroy itself?

You can use RBXScriptConnection:Disconnect() and connect it again.

local connections = {}
events.SampleEvent.OnClientEvent:Connect(function(sampleModel)
	workspace:WaitForChild("OnPlacementEvent").Value = true
	
	for , unit in ipairs(workspace:WaitForChild("MainZone"):GetChildren()) do  
		unit.Main.SurfaceGui.TextButton.Visible = true
		
		local connection = connections[unit.Name]
		if connection ~= nil then -- if MouseEnter is still connected
			connection:Disconnect()
			connections[unit.Name] = nil -- removes connection from the dictionary
			connection = nil
		end
		
		-- creates a new connection/connects it again
		connection = unit.Main.SurfaceGui.TextButton.MouseEnter:Connect(function()
			sampleModel:PivotTo(CFrame.new(unit.Main.Position.X, unit.Main.Position.Y+1, unit.Main.Position.Z))
			
			for _, part in ipairs(sampleModel:GetChildren()) do
				if part:IsA("BasePart") then
					part.Transparency = 0.5
				end
			end
		end)
		
		connections[unit.Name] = connection -- adds the connection to the dictionary
		
		unit.Destroying:Once(function() -- when unit is about to get destroyed
			local connection1 = connections[unit.Name]
			if connection1 ~= nil then
				connection1:Disconnect()
				connections[unit.Name] = nil
			end
		end)
	end
end)