Disable @all for all users in all groups and projects

In Gitlab v9.5.10, a user accidentally used ‘"@" all’ in an issue comment. This had the adverse affect of marking all 1500+ members of that Gitlab instance as participants of that issue. The user quickly realized his mistake, went back and edited his comment after submitting it, however Gitlab still shows the 1500+ participants.

1 - The page now takes in excess of 30 seconds to load as Gitlab figures out how to handle all of the participants. Is this normal or an issue without instsallation?
2 - Is there a way to get Gitlab to recalculate which participants should actually be participating in this issue.
3 - Is there a way to disable the use of '"@" all in Gitlab?

Resolved. Seems I did notice that the same person had included another @ for a group that also had 1500+ members. Deleting ‘@ all’ and the other ‘@ group’ from the comment, resulted in Gitlab rebuilding the participants list automatically. Learned something new today. Apologies for the false alarm.