Deleting forced Custom notifications

Good day

We have icinga2 on centos6 in use. This works well so far. However, I have two checks that the DownTimeStart and DowntimeEnd receive notifications. This only happens after I have sent a “Send Custom Notification” via the Web-Gui with the check mark “forced”.

I already have it with the “Reset modified attributes” command to undo this. Unfortunately without success.

So how exactly can I disable these notifications?
I have Web as well as Centos admin or root rights.

Thanks for your answer.