Is your problem that it's closing the connection? Or that it's specifically closing it with 10053?
For the former problem, there's really no way around this. If you get flooded by a botnet and you don't have flood protection enabled, you will disconnect. One thing you can and should do is enable server-side flood protection if available (see your server info for instructions, it depends heavily on the network) or use mIRC internal flood control. Realize, though, that you can't be 100% safe from floods. It is not a bug that you disconnect from a flood.
For the latter, I would say it's not so important why 10053 is your disconnection message. In fact, to the server it's probably closer to "Max SendQ exceeded", not 10053. 10053 is just a generic error that displays when a connection is abruptly cut off without a graceful TCP connection teardown. The disconnection is expected (see previous paragraph), therefore "how" you get disconnected isn't really the issue anyway.