hi Alberto,
It is not possible to prevent the new alarms coming in if a ServiceNow ticket was previously created.
However, you can prevent the display of alarms if a ServiceNow ticket is linked to it using a Filter in the IM Alarm console.
Something like:
Field: custom_1
Value: ![INC[A-Za-z0-9]]* (regex for ServiceNow Incident #, so to not "!" show anything with a Service Now ticket).
As for the emails, that may depend on how you configured the emailgtw probe.
Is it emailing on every alarm or only on assignment?