Don't bother trying to contact Grow Diaries.. they never respond to anything. The most you can do is to hit the "complain" button and hope someone on the other end does something. And no, you can't limit the number of comments you get from an account. Nor does blocking do anything! If you block someone who is harassing you, for example, all that does is prevent you from seeing what they're writing/posting... they can still come to your diary and make their nasty comments and the whole world will see them except for you.... It's completely bizarre and a stupid way of doing things - but Grow Diaries doesn't really care, sadly..
There was one person who created two accounts to harass myself and one other grower during a contest... his fake accounts actually made it look like WE were the ones saying terrible and horrible things... his account pretending to be ME was GrowlngGranny - substituting a lower case L for a lower case "i" which, quickly read, looked like my name... and he stole our profile pics.... Took us two months of OUTRAGEOUS behavior and CONSTANT complaining by the two of us and all our friends before he and his little fake accounts were bounced out... but then he came right back in again with two MORE fake accounts.... another month of torture (and he was in line to actually win the contest!!) and complaints before he was permanently banned.... So I feel your pain....
Chin up - try to ignore!