1. Capacity
The window you need is already committed. Nothing to do with your project. Ask about the next window.
2. Discipline fit
The work needs a specialism we do not have in-house. Taking it would mean learning on your budget.
3. Scope and budget cannot meet
The thing described needs more hours than the band allows. We will usually offer a smaller version rather than a thinner version of the same thing.
4. Content we will not make
Work that misleads, harasses, or exists to imitate another creator's identity.
How declines are worded
Plainly, with the reason. A vague decline wastes your time — you cannot fix a problem you were not told about.