Unfortunately, the only way to report a player's behavior through the system is by playing out the match. But that does not mean that is the only way to report the player. You always have the option to take screenshots of what the person is doing, and save them to your computer, then submit them to Player Support in a ticket. Even if you do not play the match, you will have the screenshots of the player's behavior and comments.
As far as the 15 minute penalty, that is issued as a result of having 2 or more dodges in a normal queue. You can read this article by Riot Support, which details a bit more about the various penalties for dodging. The first penalty is 6 minutes, any further ones are 15 minutes. If I recall correctly, the penalties do decay over time, if a player consistently plays without any further dodges (much like all the other penalty systems allow for a decay in penalties if no further infractions are earned).
I do agree with you, that it is not fair that the only option in situations like that is to dodge to avoid trolls like that. I've played many games, knowing full-well that the game is going to be miserable because of one person's actions in champ select. Unfortunately, there has not been found a viable solution to the problem at this time. Riot is aware of the issue, and has people working on solutions, just like they were working on ways to mute pings, and many of the other overhauls that have been done this season. It's not as simple as just putting in a report function in the pre-game lobby, because it would need to be done in a way that couldn't been abused by premade groups.
The only advice I can give you, is to try and not dodge any more games in the near future. You should be able to decay back down within a few months, if I recall correctly. Report players after the match, or if you feel you absolutely must dodge, take screenshots, and send them in a ticket to Player Support. Best of luck, and hopefully you don't encounter too many of these trolls.
