I like the 7 days trail interval. I was thinking along the same lines, namely that we need to enter into the reputation pool the actual activity. It's a problem commonly met in other social circles which are inflating their number of users, just because they are there. I have 5000 Facebook "friends" but I don't think 1000 of them are actually active. So just because you signed up and got to a certain level, it doesn't mean your authority / reputation may stay at the same level for ever. You need to engage and to prove you're "alive".

But I would extended the "aliveness" period to at least 1 month (i.e. taking into account votes for the last 30 days). It doesn't mitigate much the asshole problem, but it gives a much honest representation of the reputation, including engagement. The number of people engaged every week is just 30% from the people engaged over 30 days (or so I remember from watching the other day the steemd.com statistics). In other words, the core users are just one fifth of the whole active population. It will create an imbalance.

As for punctual situations, like the flood of downvotes from a specific account, there should simply be a threshold: if you downvoted 10 times in a row, you lose all your reputation / voting power. It will force the attacker to at least have an upvote every 10 downvotes, to reinstate the ability to vote (and possible many more upvotes to build some influence before he actually can have an impact with a downvote).