While spying on your opponent's clan isn't what we would call sportsmanlike, it's not a breach of any rules in Clash of Clans.

It's understandable if you feel upset about players doing this, but  Clan Wars is serious business. Some clans are prepared to go to surprising lengths for victory. Be careful about who you let into your clan because, as the saying goes, everything is fair in love and war. You can keep unwanted members out by setting your clan to 'Invite only' or 'Closed'.