Is it okay to rub Vibuthi or Tilak off one's forehead? Does it amount to insulting god?
I don't have an internet source for this, but I have heard in upanyasams (spiritual discourses) that No, we are not allowed to remove Thiruman, Vibuthi, Kumkum purposely. If it happens by accident, that's fine, and we need to re-apply.
In modern world, we don't want to show to others our religious markings, so either we do not apply them at all, or we apply them in morning after bath, and erase them before leaving to work.
But a compromise is to apply it faintly, and not erase it. Over course of day, with sweating/oil on face, it is only visible for acute observers.
Another name for the things we apply on forehead is called - Namam (or name), because they are applied by saying God's name. So, even if actual chandan, tilak or vibuthi is missing, you can use sand, or water, intone God's name and apply it. You can do this if you think the original will be too visible.