Dd is 6 and I know this is probably typical age related behaviour and just a phase but it's so infuriating.
She goes through phases of a few hours where she disagrees with everything. For example:
Me: its a nice sunny day today isn't it dd?
Dd: no it isn't
(while shielding her eyes from the bloody sun!)
Me: your dinner is on the table dd
Dd: no it isn't
(while looking at it)
Usually I just stop the conversation rather than argue back or say 'let me know when you'd like to talk without argui'arguing' but it means every conversation is cut short.
She has always been quite headstrong and awkward but increasingly is just ignoring things to do things her own way. For example, last night she was sitting in the hallway with her legs outstretched. Toddler dd2 asked to get past and was ignored. Dd2 tried to climb over her legs and dd1 raised her knees so she couldn't. Dd2 tried to go under her legs and she put them back down. I asked her to please let her past but she continued being awkward until dd2 cried. When I said her behaviour was unkind she said she was just 'doing stretches' 
There are many instances like this per day and I tend to tell her why her behaviour is unfair then walk away so she isn't getting more attention but still it continues so aibu in what I'm doing and what would you do?