Having multiple accounts is not abuse unless they are used to exploit/abuse the Hive ecosystem.

In order not to miss the point, let me say the following:

1. Spamming is a frowned-upon social practice.
2. Spamming translates into abuse of blockchain resources.
3. Repeatedly commenting on others' posts to insult them is spam.
4. Having multiple accounts to do what is stated in premise 3 is propagating and reinforcing spam.
5. The user in question commits what is stated in premise 3 and 4 at least in this chess community.

Conclusion 1: the user in question is committing abuse.

Spamming username mentions is often frowned upon by the community, and sometimes end up in the posts being downvoted by the community.

Yes, "spamming username mentions" triggers that because it is spamming. Username mentions that are not spam cannot trigger a spam penalty.

On the other hand, I mentioned @hivewatchers because I thought it might be of interest to them. I assume you work with them. The hiver in question gets downvotes from spaminator frequently.