I don't think there is currently a way to disable that line, but it could be an option (literally, adding a configuration knob). PRs welcome
According to `git grep` in the current instant-messaging-plugin master sources, there are two "Yippee" lines:
`src/main/resources/hudson/plugins/im/build_notify/Messages.properties`: `SummaryOnlyBuildToChatNotifier.BuildIsFixed=Yippee, build fixed!\n` – this is a text resource leading to its usage in src/main/java/hudson/plugins/im/build_notify/SummaryOnlyBuildToChatNotifier.java
a verbatim line in src/main/java/hudson/plugins/im/build_notify/BuildToChatNotifier.java apparently for a message to developer who contributed to fixing a job's state since the last (failed) run
In any case, there are several sides to the original problem (custom format strings effectively) and to Yippee's : there can be a text resource change that is relatively easy to convey at run-time through configurable instant-messaging-plugin options (global or in the job, and maybe have to be exposed in protocol plugins like ircbot), and a logical change (passing needed Java values instead of the placeholders configured in the format string, enabling or disabling Yippee's optionally) that has to be coded, compiled and distributed.