Since the last release we fixed a couple of issues related to memory leaks.
We tested your scenario with RadRichtextBox in a window with spell checking enabled with the latest internal build and didn't found any memory leaks. I've attached the sample project with which we tested.
However you scenario is probably more complicated than this. One thing that we noticed form the screenshot from the profiler you've sent is the RichTextBoxContextBehavior class which holds the reference to the DocumentSpellChecker instance that the leaked RadRichTextBox control is using. Is this a static instance? If this is the case you, can detach this behavior when the RadRichTextBox control is unloaded or simply setting its SpellChecker property to null. This will force the rich text box to detach form the DataChanged event which is most probably the reason for the memory leak.
Please don't hesitate to contact us again if this doesn't work
the Telerik team