No, don't say it. We'll understand what you want to say, but the sentence is incorrect and weird. "Service" is uncountable, so it's not possible to say "a service" here. "Obtrusive" is a formal word. "Do not" is also a formal construction. However, you're talking about feelings, so it's far more natural to say "I don't like pushy salespeople." Leave the formality for academic discussions.

Hello Egor, Here 'service' is uncountable so you need to drop the 'an'. "I do not like obtrusive service in a shop." You could also say 'I do not like being pestered by shop/sales assistants in a shop' Hope this helps Bob

A little bit better to say: I do not like obtrusive service in a shop. (No 'an') --This is more general. or I do not like an obtrusive salesman/saleswoman in a shop. --This is more specific. Enjoy.