Bot Sentry is a Pidgin (libpurple) plugin to prevent Instant Message (IM) spam. It allows you to ignore IMs unless the sender is in your Buddy List, the sender is in your Allow List, or the sender correctly answers a question you have predefined.
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Tools | Plugins | Bot Sentry | Configure Plugin
Tools | Plugins | Bot Sentry | Plugin Details
When Bot Sentry detects/changes an incomatible preference