My decision is completely final! is badly phrased. If a decision is final, it’s final, end of story. There’s no completely about it. Are-they-or-aren’t-they properties like this one are called absolute concepts. Intensifying adverbs (like completely, very, and highly), comparatives (more and less), and superlatives (the most and the least) are generally illogical with absolute concepts.

A few other examples of absolute concepts are perfect, round, straight, and unique. This last word properly means “one of a kind.”

Another option for solving problems with absolute concepts is to find a different adjective:

The other kids thought my lunch was very strange.

Note, however, that limiting adverbs are usually fine with absolute concepts:

The other kids thought my lunch was almost unique.

My decision is nearly final.