From memory, and it's yonks since I last used a floppy, there should be a little 'slider' in one corner of the floppy. sliding this will make it write protected, thus not able to be written to until you move the slider back again.
Look at the floppy from the front. Top left corner should be a cut-out. If you can see through it the disk is unprotected. Turn it over and there will be a slider. Pull it down to cover the hole and it is now protected. You won't be able to save anything else on it but you can still read from it.
Sorry, I told you a lie. Just had a look at a floppy and the cut-on the left should be see through. The cut-on the right should be blocked for unprotected, so slide the tab up so that you can see through it. It is now protected.
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