Under BCP/System Configuration/Account Configuration there is Must Surf Requirement and Days Passed to Become Inactive

These two have similar functions but operate very differently:

Must Surf Requirement
Enter the number of days between surfing before a members URLs are deactivated. This function runs automatically and needs nothing other than the member to stop surfing for this period of time for his URLs to be deactivated. This only effects the members URLs and no other feature of their account.

Days Passed to Become Inactive
Enter the number of days between surfing before a members Account becomes inactive. In this state a member does not receive any account benefits such as referral bonuses and they do not receive hits to their URLs. However this feature is not automatic. It is only run when you run the System Optimizer and you have to choose the Member Inactive Notification option. This option is intended to remove the "dead wood" URLs from rotation and keep the traffic flowing to your regular members.

How to use these options
If you do not want one or both of these options then enter 0 for the value.

Experimentation with your membership will provide best results but as a rule of thumb keep the Must Surf Requirement to a lower figure such as 7 or 14 days and the Days Passed to Become Inactive higher to something like 60 or 90 days.

Note that no accounts are deleted and at anytime a member can reactivate their account by surfing the requirements you set in the Account Configuration.