Ideas and suggestions.

There's a great (and most importantly, simple) solution: just remove all coefficients for all templates, and arenas are not really "Heroes" matches; there shouldn't be a rating for them.

Here, it's one of two things: either there are no coefficients, and the reward for an arena or M200 match is the same as for an XL+U match. Or, the coefficients already exist in some form (and it's impossible to distinguish an arena from a non-arena match using automated means, so the filtering must occur based on other criteria).

The idea of reducing the coefficient in games without rules seems reasonable and quite simple to me.