For example, to become a Hero member, a user needs to earn 500 merits. On the other hand, he also has 250 spendable merits. On the current system, even a user who doesn't spend any smerits will rank up. So we can do something on that. Users must be forced to spend their smerits as well to reach the next rank. If he/she needs 500 merits for Hero rank, he/she also needs to spend 250 merits to achieve Hero rank. I don't think it's necessary to add any additional option for user profiles since each user can see how many spendable merits they have. This is my first suggestion; if theymos think it's reasonable, he could implement it.
Another idea is to get back merit rewards. Like spending 2 merits, the receiver will get 1 spendable merit. And the sender will get back 1 merit to their profile. And both should be spendable merits. But if this is implemented, users will reach fast to the next rank. Then the merit limit for rank-up should increase as well. For example, Hero members need 500 merits now; if this is implemented, they should have 750 merits for Hero ranks. That's how it could be balanced. But I feel the first idea is better. No need for many changes to the forum software.
Feel free to drop your suggestions as well.
My thoughts does appeal with your first suggestion unlike the second which can be easily exploited by some supposedly smart users.
On the first, which suggests that users must have to spend half of their total required merits to any rank. That's thoughtful . But I think that with the alleged issue of merit trading from some quarters in the forum, users can still be able to defeat such suggestion if implemented.
Maybe a S'merit perishing within a 60 days duration of it unused can deter users from hoarding their S'merit.