Players are free to accept or decline match invitations, regardless of rating.
However, deliberately using another nickname to artificially adjust or protect a rating is a different matter. The FlyOrDie Rules specifically include “artificially adjusting your rating (resetting, banking, playing another of your other nicknames ...)” under cheating.
If you believe specific players are doing this, please report the nicknames concerned together with any relevant evidence so that the activity can be reviewed.
Refusing a match alone is not evidence of cheating; there would need to be evidence of rating manipulation or another breach of the rules.