How can I remove the windows partition?

Start Bootcamp Assistant and select the option to remove Windows and return your drive to a single OSx partition.

Do not use Disk Utility.

If one, mistakenly, had used Disk Utility to remove the partition and now was stuck with a useless partition, how would one go about erasing that partition?

By erasing the entire disk and starting again, there is good reason why Disk Utility is not recommended.

Hi,

I have kind of the same problem: I installed Windows 8 on a partition (which previously contained Windows 7) but removed it again; Windows 8 was for some reason booting automatically; now it is removed, I still have to press alt at startup to select OS X, because otherwise nothing is booting up at all; I also used Disk Utility to remove the partition instead of Boot Camp itself, so maybe that's the problem?

If I try to make Mac OS X the default startup disk using system preferences, that doesn't work either.

So, might it be a good idea to indeed erase the entire disk and start all of it again? And if so: can I do that just by using Time Machine?

Thanks in advance!

Yes, start over, do it right (which means no disk utility) or do it properly by buying a Windows machine and forgetting about this nonsense.

Test your backup before erasure.