Just for clarification:
If the square tabs at the top of these cassettes have been removed (or are slid inward towards the center of the cassette), then the cassette was originally intended to not be recorded over.
If now, you wish to re-record onto cassettes where the tabs have been (re)moved, the others are suggesting that a strong piece of tape (like duct tape) could be used to re-cover those holes and make the cassette able to record once more.
If the tabs are still in place, then you can re-record over them as often as you wish.
-Wolf sends