Q. In this age it is very difficult not to think anything if you have seen or hear or feel something wrong from a Vaishnava.
How to avoid this aparadha?

A. Yes, it is true mind can not remain inactive in its function of thinking, feeling and willing.
We are pardoned as long as these subtle activities of the mind are not manifested on the grosser level by speech or by action.
If any negative thoughts come in the mind, we can replace it by positive thinking about a particular Vaishnava.
Everyone has some good quality. If you simply focus on good quality and magnify it, then the negative thoughts may not come.
You may not speak to that particular Vaishnava, however you may glorify him to some another Vaishnava and in this way this ‘anartha’ shall be removed.
This is an austerity of mind and speech.