Following are the steps to disable the setting:
1. log into firewall locally;
2. change the URL of firewall to following:
https://(firewall IP)/sonicui/7/m/diag
3. click on 'Internal Settings';
4. see screenshot below, search for 'preempt', then disable the setting:

5. scroll to top, click on 'Accept' then 'Exit Internal Settings':

Now users won't be preempted by NSM.
Note:
The setting needs to be enabled after. Or it will cause communication issues between NSM and firewall.