Is there a way to remove the cheque numbers from an account on mass.
I tried selecting all the transactions, right click and edit. I can change the cheque number to a different number on mass, but not delete it.

For now, I don't see a clean way to do this via the GUI. You can mass fill the number field with a single blank but I don't oversee the consequences that this might trigger in the long run.

Of course I can imagine a few alternatives how to achieve what you want, but they would require some knowledge on tools and courage. I mention them anyway, but leave it up to the you to follow any of the paths depending on your skills.

a) Save your data using the database backend and do the modification using a database tool and convert the database back to a KMyMoney file. The first and last step can be performed using KMyMoney's Save As... feature. In case you store as sqlite database, sqlitebrowser can help you editing the data.

b) Convert the data file into a readable/editable format. See the online manual how to do that. Then use tools like sed, awk, perl, python, ... to make the modifications. That is the tricky part. No need to convert it back. Simply rename it and KMyMoney does the conversion when saving the next time.