Currently the NPoS is open to griefing & PoS attack by placing a lot of entries into a single validator's nominator list.

One mechanism against that would be:

• force_unnominate (for a validator to unilaterally remove a nominator from them); and
• max_nominators (beyond this no more nominators could nominate the validator).

Could also introduce a set_max_total to introduce a maximum amount beyond which nominators' balances make no difference.