I disagree with sanctioning Mods who withdraw from multiple projects.
There are many real-life occurrences where you might be forced to do so, and from many projects all at once.
However, if you see a pattern that one user does that a lot, naturally you as a CM will not give him that position anymore. That would be the proper punishment.
Rather than punishing/sanctioning/limiting all the time, we should think of rewarding the good behaviour. Create privileges for projects which were completed by a user.
For example, Viki can create Moderator badges indicating 5, 10, 20, 50 and so on (MOD5, MOD10, MOD20, MOD50) finished projects. This would be particularly helpful for CMs when hiring.
- It would identify really fast which users quit their projects midway - those with many, many projects in Project Contributions list and a badge indicating just 5 finished projects.
- The CMs will know that applicants with higher Mod badge number are on average (but not always) more likely finish the project.
- It will give a subconscious incentive to the Moderators to finish their projects the same way as a subtitling badge does.
These badges may also carry more practical privileges, such as having a higher number of allowed projects once you reach 20 finished projects.