They might be bots making automatic signups for the sole purpose of posting their site's URLs around the net. I had problem with bots posting porn sites. I even required admin authorization to activate but their profiles still listed in the memberlist, which is all they care about anyway.

I would suggest you update to the latest version of phpbb for 2 reasons. The most important is there is a vulnerability that is causing a LOT of sites to get hacked. The other reason is this version includes visual confirmation by default, instead of a mod you must manually install. Since bots can't "see" the visual confirmation images, they would fail the test and can't register. I haven't had illegitimate signups since implementing the test.

You need to take off those extra dark sunglasses Coon