For steps to clear the stateless alerts manually you can refer to below MOS notes,
EM 12c: Removing Multiple Incidents at once in the Enterprise Manager Cloud Control 12c Incident Manager or emcli (Doc ID 1511659.1)
How To Delete Multiple Incidents In Enterprise Manager Cloud Control 12c (Doc ID 1454960.1)
Additional reference: 12c Cloud Control: How to Configure Incident Rule to Automatically Clear Stateless Metric Alerts? (Doc ID 1559436.1)
I executed this following command trying to preview the stateless alert of db1 and total alert shows 0. However, in OEM console there is indeed an incident of db1 because of a Generic Alert Log Error. Did I do something wrong here? Or is target_type not oracle_database? Thanks.
./emcli clear_stateless_alerts -older_than=0 -target_type=oracle_database -target_name=<db1> -ignore_notifications -preview
The following alerts can be cleared:
From Console you can try the following
Login to Console
Under Enterprise->Monitoring->Click on Incident Manager menu item to go to the Incident Manager UI and then click on "All open problems" link on the left
Identify the specific problems for your target.
Click on the problem link to show details
On the Right Hand Side, you should be able to Clear the problem which will clear all Incidents associated with the problem.
Navigate to the Target Homepage
Click at the target level menu and navigate to Monitoring -> Incident Manager => Click All open problems on the left and identify the problem from there -- Click on the problem link to show details --> You should again see the clear option enabled to clear the incident here
Also you can refer to below metalink doc to understand which metrics in Cloud Control can be manually cleared if required
What are Statefull and Stateless Metrics in Enterprise Manager - Explanation and Example (Doc ID 733784.1)