Ah, I see now. You should be able to modify your addition (see at ***, below). I'll try to answer both your general and specific questions.
MapShare entries should not be undertaken lightly .. as you'll understand after the following explanation.
First, when you enter a correction to an existing map feature or built-in POI category, you'll be asked if you want to share the change with the community. If you do not, the change remains local to your unit. If you do 'share' the change, the information will also be retained on your unit, but in addition, the next time you connect your unit, the change information will be forwarded to TomTom for review and possible redistribution to other users.
Once the change exists on your unit, depending upon the nature of the change, it can be modified or perhaps deleted. For example, if you add a turn restriction at an intersection (e.g., indicating a 'No Left Turn' that was missed), you can go back later and effectively (but not actually) remove that turn restriction by going back to that same intersection and indicating that a left turn is allowed. On your unit, you will see only that the turn is now allowed. If you also 'shared' that information, TomTom will see two entries ... the first one that indicates the turn is not allowed, and a subsequent entry that shows that the turn is allowed.
*** In the case of a POI that you have added to an existing TomTom category by using "Map corrections", you can return to Map corrections / Correct a map error / and use Edit POI. From there, you can 'Delete' the POI, move it, change (or add) a phone number, rename it, or whatever.
Last ... if you perform a Factory Reset on your device (done from Settings / Factory reset settings), all of your Mapshare changes to roads, POI, etc., are deleted. IF you do so before connecting your device to your PC, those changes will also never be sent to TomTom for evaluation, and as a result, not only will you not see them on your unit anymore, no one else will see them, either.