They did not keep anyone from voting. Failure to understand the rules did.
The rules were that the delegates in CO would not be bound going into the national convention. So, right from the get-go, the people of CO had no choice. Given those rules, which were in place since last August, Trump made the strategic decision not to campaign there. You can't blame him for shunning CO, when the rules made campaigning there pointless. I do not know what happened so that suddenly, the CO republican party insiders (not the voting public) all of a sudden decided to bind the delegates--obviously, the situation is extremely irregular. Was the Trump campaign even told about this sudden decision to bind the delegates, or was it told in time to prepare? I doubt it. This was a naked delegate-grab.
Now, what I cannot know is how much input Cruz had into this delegate assignment process. Maybe he was not intimately involved, and he'll just happily accept the result (he is a lawyer, after all, a member of a profession for whom the ends justify any means). However, it reflects really badly on him. If he was not complicit in what happened in CO, the most honest thing he could do is distance himself from it and denounce it, while making some statement about supporting the will of the voters. But he isn't honest, is he? It is this dishonesty, as much as anything, that has turned people like myself, who once did not care much one way or the other about Trump vs. Cruz, into firm Trump supporters. No way I could ever vote for Cruz now.