doing a emctl clearstate does the same thing as well, i ended up raising a TAR on this, and the reponse was:

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Hello Barry,
that's true. Old alerts somtimes not cleared. This problem is addressed for thenext major release (10.2) In which release it will be possible to de
lete the alerts manually.
At the moment, please try the following:
Go to the node the alerts come from.
Stop the 10g agent on this node and run a
emctl clearstate.
This will drop all the alerts for tha agent.
Please wait ater the restart of the agent some time. SO the OMS and the agent can synchronize.
Regards,
Bernd
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Totally impractical.

Another example how functionality has been lost from the older OEM's in order to rush out Grid control.

Bazza