If I understand you correctly you want to ignore all data in the receive buffer?
If you NEVER need any data received, just don't script a on sockread event for that socket.
Or just return from the event without doing any /sockread -> mIRC will think there's no handler and clear the buffer. If you already read some data, just return, let it retrigger the event and then return without /sockreading.
Another way is /sockread &a and then just do nothing with it.