I have a system I use. It doesn't matter if they are Dis or not. If I don't see them within a month or even two, I delete them. If they do nothing but ask for free things or send messages with 1 word, ex. COME, I delete them. If they enter rooms I am in and never say hello, it's obvious they are trying to collect other CL's or staff, I delete them. If they have been banned for a legitimate reason, such as fighting with others, they get only 1 more chance to change or I delete them. There are some on my list I keep, solely to watch for a while. These are the trouble makers that I will not hesitate to report. So far, none seem to be Dis members, but you never really know for sure.
Just because someone is a member of the Dis, it doesn't make them a friend. If we don't get along well, why keep them on my list?
Of course, there are a few Dis I keep on my list that play vmk at different times than I do, I try to keep them on so that we can visit each other, time permitting.
The friends list is there for you to keep in touch with your friends. Not to be filled up with anyone who requests friendship. Don't feel guilty about deleting people from your list. Remember, your list has only 150 spots to fill. Sometimes it's a necessity to delete. Once all of those spots are filled, that's when it becomes hard to decide who stays and who goes.