The Root Problem

Anyone who can use Differential can create a diff and a revision and there is no Default Edit Policy for revisions, so on installs where task creation and editing are restricted, spammers will target Differential revisions and may create trash diffs.

The Solution

That's why it would be nice to add the ability to change the Default Edit Policy for Differential revisions as well as restricting who can create diffs (for revisions) by adding a Can Create Diffs policy.
An example use case would be to restrict the ability to create diffs and edit revisions to Trusted Contributors on this install.

A similar approach to Can Create Diffusion Repositories and Default Diffusion Repository Edit Policy can be used to implement the aforementioned policies for Differential.

The Benefit

• Can Create Diffs: Prevent possible spammers from creating diffs that would be difficult to abandon if differential.always-allow-abandon is set to false
• Default Edit Policy: Prevent spammers from vandalizing on revision titles, tags, subscribers, summary, test/revert plan, blame rev, etc.