Is it possible to restrict access to users with the free wiki so that they cannot edit the page?

Yes.

You can allow users to see your wiki, but not edit it by doing the following:

1) Make sure that your wiki is public. To do so, click on "Settings" and then "Public/Private" under Access Controls.

2) Set a wiki invite key and do not give it out to users. If you've already given out the wiki invite key, you can always change the invite key to block further edits. To do so, click on "Settings" and then "Keys and Access" under Access Controls.