I assume when you mean remove their stuff you mean they leave items like bikes or something similar in public areas. Well I had this problem with my neighbour who left his push bike and usually fell over it when leaving my house. Two ways, either ask them nicely but if you're unsure, ring the fire station (not 999 tho) and they do free fire hazard checks, so they come to your area and tell you whats wrong because if you had a fire and tripped on your neighbours things and knocked unconcious because they left something near a door or stairs etc. firemen will be mad.

I would suggest biting the bullet and politely asking your neighbours to remove their items. You do not need a reason to ask as it is usually against the rules. If after a few days the items are still there, suggest to your landlord/managing agent, that you feel the items are a fire risk and ask them to write to the neighbour. The agent will usually have the right to forcibly remove the items if necessary.