A materialized view create with the automatic refresh can not be alter to stop refreshing. In order to do that you must broke the dbms_job that was created in order to refresh the view.
Connect as mview owner.
select * from user_jobs where broken ='N';
In the WHAT column for the mview refresh job you will see :
Take the JOB_ID form the job column and execute:
And the mview will stop refreshing automatically.
If you want the mview to start refreshing again just run the job.
Then the mview will refresh and continue refreshing by the interval.