Light filters and challenge rules

If a challenge host wants to limit post processing, then chances are they may also want to limit the use of some light filters that attach to camera lens. Of course their rules limiting PP do not apply. What is the simplest, most straightforward, most effective rule they could state to bar filters that alter the image so that it is changed in some way from what the photographer sees? I know very little about light filters but I assume they should not say "no filters" and that it is not feasible to list specific filters.

There is a current challenge that limits PP, but there is an entry of an IR image. Now we don't know if the host cares about that or not, but if the host did care there is no recourse since no rule was broken.

Please note that discussing the pros and cons of filters and PP is interesting, but does not address the question.

Tim