Granting execute permission to all procedure with select query

i have created a user in my sql express and granted read access to all tables.When i try to edit table data using this user then server throws error saying permission denied.
I granted execute permission to this user, and tried executing procedure which changes table's data then change is committed to database.
is there any way through which i can prevent user to commit change in my table even through stored procs?
Please helpRegards Kumar Gaurav.
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