The USDA Agricultural Research Service (ARS) has seen promising results using 1 and 2 percent caffeine solutions to kill snails and slugs. Research showed the 2-percent caffeine solution more effective than metaldehyde, a common pesticide used to control slugs and snails.
Instant coffee is about 0.05 percent caffeine. Normal brewed coffee is a little stronger. The coffee I make is defiantly strong enough to kill a slug. Though I have no idea where you find coffee cups that small.