Soka Gakkai International (SGI) is a Nichiren-inspired lay organization. Their membership fluctuates. This is in part because many people join only to leave not long after joining. I am not interested in discussing the reasons why someone might want to leave SGI, or in arguing that they should. What I would like to do is answer the question What ought I to do when I want to leave SGI? in a place that is public, readily findable by google search, and rebuttable by anyone willing to abide DharmaWheel's terms of service, since this is a situation in which many convert Buddhists seem to find themselves. While I recognize this is a controversial subject, I hope that raising it in this way will be productive of constructive dialogue and not disharmony within the Mahayana community at large.
So, to the question: What ought I to do when I want to leave SGI?
If you have good reason to leave, then leave. Leave on terms of friendship and goodwill, but do stop attending services and events, and stop practicing Buddhism for a time in the way you learned at SGI. You have choices.
It may be that your experience in SGI has completely soured you on the idea of practicing any form of Buddhism. If this is the case, please hear me say that Buddhism is a very diverse world, and that the form of practice and doctrine taught by Ikeda and others at SGI is not representative of that diversity. Some have argued that what SGI promotes is not really Buddhism at all, which is an argument I will not enter here. The main thing is this: please consider shifting to a different approach to Buddhism, as a way to salvage the best of your SGI experience and to keep some continuity in your spiritual life, and for other reasons that should become clear to you as you practice.
It may be that you are confident in the teachings of the Buddha Dharma and the Lotus Sutra in particular, but have lost trust in SGI as an institution. I have good news for those in this position: there are plenty of other traditions of Buddhism that emphasize the teachings of the Lotus Sutra, recite it and venerate it, in different ways. Tendai is one. Any of the other Nichiren schools and organizations (such as Nichiren-Shu or Rissho Kosei Kai) are also available and welcoming. If you have faith in the Lotus Sutra, by all means keep the faith.
It would be healthy for anyone on the way out of SGI to expose himself or herself to a variety of Buddhist traditions. A discussion board such as DharmaWheel is a terrific resource for helping you find authentic practitioners and teachers in your area, thus saving you time and sour situations. It will also help to study broadly in Buddhist teaching and history, so you understand better how SGI fits into the spectrum of Buddhist practices, and to see more clearly where your own future and interests might lie. (Paul Williams' short book Mahayana Buddhism is a good place to start, one of many.)
The experience of parting from this organization, like any separation, may prove to be traumatic for you. Beware of this. Resources are available for people in this position. You aren't alone.
Once you find your feet and your sangha, the question will arise on how to relate to those you left behind in SGI, your former comrades. This is a question I will not presume to answer. Let the Mahayana be your guide on this.
[note: my comments here summarize several emails and discussion-board PMs I have sent over the years from a number of people who should and will remain anonymous. Posting this thread here may save me keystrokes in the future in similar private conversations, but more importantly, it will allow others to chime in, right me where I'm wrong, and contribute as helpful.]