Here's a substantial revision of my original answer. After waving my arms in the mirror and reciting for an hour, I'm convinced I'm right. I'd be very interested in any comments.
Merit refers to a person who has won merit, in other words, a man with great accomplishments.
The unworthy thus refers to men who have either no accomplishments or even faults. 'Spurn' is literally 'to kick', so the sentence means "the abuse that men with great accomplishments receive from men with none'
The word 'patient' is crucial here. 'Patient' can be positive, in the sense of 'steadfast' but it also has a sense of 'long-suffering' and this can be quite negative. There is an example of this in Coriolanus:
First Conspirator. Your native town you enter'd like a post,
And had no welcomes home: but he returns,
Splitting the air with noise.
Second Conspirator. And patient fools, 3880
Whose children he hath slain, their base throats tear
With giving him glory.
I suggest that patient here also has this connotation: long-suffering to no purpose, passive and without spine. It is thus a form of irony; you could put scare quotes around it.
I think that this interpretation is supported by a deliberately ambiguous use of 'take'; this is usually brushed off as 'receive', but here I think it also suggests 'to accept'; in modern English, 'to put up with'.
I think the overall structure here also favors this view: it is the last, and most outrageous, of a list of insults and humiliations ('slings and arrows') that are hard to accept, that Hamlet tells himself he won't accept, and as the most outrageous, and least acceptable, it justifies the action proposed in the very next lines: "When he himself might his quietus make/With a bare bodkin..." Are you going to be a sucker? A fool? (raving) Or be a man and do something?
Anyway, my 2 cents.