I saw you found A solution but maybe not the one and only.
So what’s happening is that, the clients will not automatically deregister which is an intended behaviour since otherwise you’d not get any sort of “keep alive’ alerts.
Moreover you might at some point just be willing to check if a check has not be running for a while, like we do but those alerts will hang around for example also if you just tested a check and never executed it again.
You can thus just go to the corresponding client you have an issue with on uchiwa and delete it from there.
You can perform the same action with a curl request against the sensu-api.
The advantage over hitting flushall
is that you will only delete the client(s) you want to delete whereas flushall
will also remove all your stashes which might be very annoying at a larger scale.