ISAM memory usage when running online for a long time

[ジョンスミス]

I want to use ISAM to implement a localization fusion feature with GNSS + ODOM + some static environment feature (like a small SLAM). The application is designed to run online in vehicle for long drive distance. 

Wonder will the memory usage keep increasing because i keep adding factor into the graph. Since actually there is no need for loop closure in this application, actually those factory that far from current position is useless any more. Do i need to remove these factors  and states from graph manually and how should i do it?