The "HRT..." is the model name, but the
HRT216S3DA does
not have a blade clutch (yellow button).
Now the "MZCG-" is the exact serial number, but it matches to a
HRR2163VXA, which
does have a blade clutch, so we'll go with that one.
Anyway, that serial number is in the range of some mowers that were part of a Service Bulletin issued back in September 2006; the issue is the blade control may release even though the customer does not let go of the the lever. The problem is likely due to wear of a pivot cam inside the blade control...the fix is to replace it with an updated design. For mowers still under warranty, this is done at no charge...FYI, it appears your mower was probably purchased in May/June 2006 (2-year warranty).
Any Honda dealer can do this job, for probably 1 hour or less of labor. If you prefer to DIY, you want to get Honda part number 75180-VL0-B00, and drop me a PM and I can send you instructions. Please include your name and address, and I will update the registration database to show you as the owner. Honda asks for this in the unlikely event there is ever a product safety recall and needs to contact you.
Google any part number to find a Honda Dealer selling it online, or use this link to find a Honda Dealer in your area:
Find A Honda Dealer