Members getting auto pruned

Probably a newbie thing . I have searched help, Faq's and forums without answer - So I must be doing something reallly silly ...

My problem is that members are getting thrown out of the forum after a few days just for not visiting. THe email they get is ...

Your account on "******" has been removed by a administrator, this is done according to the site account policy
probably due to inactivity.

Well I'm the administrator and I didn't remove the account . I guess there is some setting somewhere but I don't seem to be able to find it.