From answers to this question, it would appear that some Canon cameras can perform a 'Low Level Format' on SD memory cards, and that this will write data to the whole card (i.e. overwriting the free space).
It's only available for formatting (not deleting individual photos), and only on some cameras, and only on their SD cards (e.g. Canon 5D Mark III only offers it for the SD slot, not the CF slot).
It's a feature I thought was only on the DSLR range, but it appears several (if not all?) Canon PowerShot cameras can do it, so would expect to find it on a few other compact Canon cameras (perhaps even other vendors). For example, S110, SX280HS, even the rather old A550, so I suspect most Canon cameras with SD cards will support it.
Also worth noting, according to this discussion thread, that some cameras' low level format doesn't seem to securely erase as expected. One user found images could be recovered after performing a low level format with their 450D, but not with their 70D. It's an internet discussion forum, so take with appropriate levels of salt, and definitely test any secure erase/low level formatting thoroughly before you rely on it!
Finally, as various others have commented, if you truly need secure erasing, then there's nothing better than physical destruction of the card. I certainly wouldn't guarantee (even with thorough testing) that a low level format will properly wipe the card. It may be good enough for most purposes, but you still have risks (like the chance some sectors don't get erased, perhaps quite deterministically, and the likelihood of a user forgetting to check the 'Low Level Format' option). You could talk to Canon about how Low Level Format works at a technical level, and if there's any guarantees on it wiping the data. Otherwise consider using small/cheap SD cards and considering them as disposable (with an appropriate protocol to ensure destruction).