Introducing a time limit wouldn't work. Many users only visit SteemIt once a day. Maybe they'll have a look around for something they like (or for something which will earn them money - I add this for those cynics amongst us :o) ) and upvote it. They are not going to wait at the computer so that the can vote again.

Again, if someone is limited to the number of votes, how are they to choose which posts to upvote and which not?

I see nothing wrong in vote spamming. If a post is good it will gain even more votes than the spammers give and the post value will also be an indication. In fact, for newcomers such as me it's quite nice to see that my posts get 42 votes, even if it doesn't get many views.