Look at the GRANT and DENY transact SQL statements in Books Online.
If you also want to restrict it somewhat from administrators, you can do several things...
a) Create a trigger for INSERT and for UPDATE - have it do a rollback.
b) You can (search on this site to find out how) mark the tables as system tables.
Both of these can be overridden by a savvy admin. I would recommend that you use the GRANT/DENY statements on a login that your app uses, or on a group that all of your users belong to.