You can try to wait until just after they steal you and before they bank it then attack them. This will end in a stalemate.
You have to hit at the perfect time. When their spies are down to maybe 20-30%, you should be able to attack them and win.
Being a pure spy with no allies, and banking till under 1k, most only get 1 hit, then dtw happens, it is truly the most irritating build when played well, a really low maintenance acct to have... My suggestion make peace
Assassinate them and keep them pinned day and night, simple yet elegant. If they are smart they'd add a t1 lvl1 atk building so you have to scout them to keep spies pinned.
Use crystals and spy equipment defense, maybe you'll get lucky to see who the spy is. Try not to be on their bad side :|
The only way you can really phase him at all is if he has pots you can burn. Otherwise just try pinning his spies. Theres not much you can take from someone who has nothing.
When they sleep, hire their allies and attack only I think anyway. I haven't checked rest of comments so this could have been said, if it has. I don't care.
Why is everyone saying to use spies? Game mechanics says you can attack a pure spy with gold out right? Give him gold, and attack. He won't go DTW as he has no troops. OP said nothing about the spy being ally less. Pure spies tend to have no allies, but it wasn't mentioned here