The OED examples since the late 18th century are consistent: “namely” is followed by a comma. In broad terms, whatever follows it is [a list of] that which is then named/described/titled, etc. The list can be viewed as being in apposition to an earlier reference and this makes for the good use of a comma.
3.a. Introducing more detailed information or a particular example: that is to say, to be specific; to wit.
1798 G. Washington Let. in Writings (1893) XIV. 99 It would then have been understood as it is at present, namely, that the gentlemen would rank in the order they are named.
1855 A. Bain Senses & Intellect ii. i. 349 A third quality of sounds, namely, cadence or accent.
1875 A. Helps Social Pressure iii. 48 The worst and most disheartening point..is this namely,—that the course of modern thought and modern life is set against these improvements.
1934 R. Graves I, Claudius vii. 110 I have mentioned Julia's children..namely, her three boys, Gaius, Lucius, and Postumus, and her two daughters, Julilla and Agrippina.
1998 J. Irving Widow for One Year 442 He'd asked her to look for some important papers—namely, a codicil to his will.
That there is disagreement is fine. The essence of the punctuation after "namely" is to provide a slight pause. This is achieved also by the close synonymous phrases in the OED's definition, namely, “that is to say, to be specific; to wit.” Without doubt, the easiest way to do this is with a comma – everyone understands a comma. A colon or dash might work, but these tend to indicate a longer pause - this is useful if that is the way it was spoken or intended.