No. Bad form.

If I find an unannounced rider on my wheel I sit up and slow right down. If they don't know better than to announce themselves I don't want them on my wheel or anywhere near me as they are clearly an unpredictable rider.

If I sit up (literally) and they roll up beside me and say hello I will engage in a quick chat and go from there. You never know. They could have buried themselves chasing you down and just hadn't recovered enough to announce themselves yet.

If they just sit back there I will just keep soft pedaling until I am crawling. If they don't get the hint they are truly a dumbass.

If they jump past me without a word I used to chase them but never close the last 5m, pushing and pushing them until they cracked. If they didn't crack then I had fun and made my point anyway.