Your schedule is a strategy whether you plan it or not. Unplanned time gets spent by whoever asks first.

Blank time is not neutral, it is available, and availability is a standing offer to anyone with a request. Whoever gets to you earliest takes it, and the things that matter to you lose by default because they never asked. The week still had a shape, it was just drawn by other people.

Put two or three of your own things in the calendar before anyone else fills it: the training session, the two hours on the essay, the evening you want free. Then the answer to a late request is genuinely that you are busy, which is both true and much easier to say.