Refering per question:
- You can't: actual links are always safe; linked resources (thru embeddings, such as pictures) aren't.
- Not only "send", also "use".
- That is irrelevant, since a phpBB installation can't do that out of the box.
- Then disable the BBCodes
flash everywhere: posts, signature, PMs.
But what exactly do you consider an "unsafe link"?