It's not. At the very least, you should pick one of the default ones if you don't need to give details.

P.S. As per the guidelines, please make the title as specific as possible. I take you should edit the title with the question itself.

Edit: ninja'd

Admins are supposed to put a block reason in block summaries.

Is that called “blocking people for no reason” if you don’t put something in the summary?

It could potentially be if the blocked user had no means to know why the block happened (e.g. not messaging on said user's message wall before or after the block).

"Disruptive editing/posting"; that's my personal quick fix summary for such occasions.

You can also add wikilinks in block summaries, as long as they connect to a policy page (Which is what they're usually for). Keep in mind that block summaries cannot be changed once published.