Instead of trying to block or circumvent the tracking pixel, consider accepting the fact that they're tracking this information, and learning to work around it.
For example, one primary reason I can think of why management would want to do this is so that they know when you've seen an email and how long you take to reply (assuming a reply is necessary.) So, when you read such an email for the first time, reply immediately. Say something like "I'll look into this and get back to you" or "thank you for the suggestion" or whatever. Then proceed with your regular work schedule and prioritize the email request accordingly, as you normally would.
The manager gets his swift reply, you openly acknowledge that you've seen the email and can (presumably) move on with your day. The fact that there's a tracking pixel involved doesn't really add any new information at this point. The emails are timestamped anyway, so even the question of when the pixel fired is redundant information.
I wouldn't worry too much about the pixel firing multiple times if you repeatedly reopen the email, until and unless management actually raises that as a concern. If they do, you could always consider temporarily disconnecting your computer from the network before reopening an email you've already opened. Or, you could copy/paste the body of the email into a text file and reference that as many times as you want.
That way, they still get their precious pixel tracking on the first time you open the email, but remain blissfully ignorant of how many times you re-check the content later. But this is all extra work for you (however slight), so like I said, I wouldn't worry about it until they complain about it.