Friendly farming are legal except when it is from multi-acc. The multi-acc should got perm ban.
And sometimes they claim to be the sitter to hide their multi-acc.
Therefore I think the solution should be to limit Sitter duration to avoid Sitter abuse. Let say max Sitting day limit (SDL) is 7days. After that you cannot sit for that acc for the next 7days =Sitting Ban Duration (SBD). If you sit again for that acc in the first 7days after the first SBD, The 2nd SDL will reduce to 4days and SBD double. 3rd SDL 1days and SBD triple. 4th time permanent sitting ban.
And if it is proven that more than 2acc play multi-acc or sitter abuse or cheating in an alliance, whole ally members will get penalty too. This is to promote honest gameplay and avoid associated with them.