There are three options when protecting content/working with RSS.
Set Hide/Show option in the Settings > Configuration > Protection Defaults section of WishList Member to Yes.
This should hide all protected content.
Set the Auto-Inserting of More Tags option in the Settings > Configuration > Protection Defaults section of WishList Member to Yes.
This should clip the Posts so that the RSS feeds only show the desired amount of words before the More Tags.
Set the RSS Feeds to display only the Summary in the Settings > Reading section of WordPress.
The above scenarios are options so you can select which of the three will work best for your site.
Keep in mind that different readers/online services may interpret the data differently. One may only show the title while another shows the title and information before the more tag.