Note: I’m not a PagerDuty support team member, so I will try to give you my thougts, but you may open a case if you want an “official” answer.
link multiple technical services to the same configuration item in ServiceNow
AFAIK: No.
When PagerDuty will send an incident to ServiceNow, how ServiceNow knows which CI to use if you mapped the service ID to many CI?
The level of detail is not the same in the 2 systems (more detailed in pagerduty).
As I said, you can map a service to any type of CI.
So what I said to our support teams: select the CI which fits your PagerDuty service (ie: the same scope)
In my company, ServiceNow CI may be: an application, a part of an application (called “a module”), or an “application instance” (which is a tuple: application or module + a type of environment like “dev”, “prod”, …)
Use your own CI types.
If they are not enough, you may create a new one. For example “Application service”. An “application service” is a child of an application, and its scope is a PagerDuty service … then it obviously good to map your PagerDuty service. 
we want to do is to de-activate the way from ServiceNow to Pagerduty
I think you can do it, but it should be via an ugly trick. (I don’t know how)
But you should avoid that, IMHO.
Example: if you resolved an incident from ServiceNow, you want it to be solved in PagerDuty (I hope you want)
Example: if you reassign an incident from ServiceNow, you want it to be reassign in PagerDuty and notifty the related escalation policy oncall guy (I hope you want)