Spyder IDE - Cancel opening large variables without restarting the entire program
(Spyder developer here) My answers:

Cancel the request to view the variable without closing the program?

No, that's not possible, sorry.

Set a default so Spyder will only display the first 1000 rows of very large objects such as dataframes?

That's already in place. The problem is the size in memory of your dataframes because Spyder needs to make a copy of them to graphically display them.
To fix this problem, we are planning to use more efficient serialization libraries (e.g. pyarrow) in Spyder 5, to be released in 2021.
I am working with large Pandas dataframes in Spyder. Occasionally I accidentally click the large dataframes in the Variable Explorer window and Spyder will very hang for very long periods while it tries to open. 
The only way I have found to stop this process is to close Spyder completely and then reopen. 
Is it possible to: 

Cancel the request to view the variable without closing the program?
Set a default so Spyder will only display the first 1000 rows of very large objects such as dataframes?