interfere, intervene
Interfering has an aggressive connotation, and intervening has a peaceful connotation (also used when intervention is necessary).
Note: These words can be interchangeable depending upon your bias or opinion.
http://www.italki.com/answers/question/62849.htm?hl=ja
intervene = to get involved in a dispute
to interfere = to disrupt in an unwanted manner
Vanessaより