• https://docs.microsoft.com/en-us/azure/data-explorer/kusto/query/sqlcheatsheet

Select records where a column does not contain any of the listed substrings

SecurityEvent
| where not(Computer has_any ("mgmt", "imagine"))

Alternate form that works for a single substring match

SecurityEvent
| where not(Computer contains "mgmt")

SecurityEvent
| where Computer !contains "mgmt"

Get a count of records based on summarizing a specified column

Event
| where not(Computer has_any ("mgmt", "imagine"))
| summarize count() by Computer