I went into the kitchen and there was water spraying up from the sink pipe.

I went out and bought all sorts of powders for Fleas, went underneath the unit, sprayed all the yard, did anything in my control.

I was going to get them professionally sprayed - well worth the money if you ask me - I've painted my fair share of cabinets in the day and it's not as easy at it looks to get really good results. 

@Sam Wilson I had the idea of spraying direct mail letters with perfume to make each one smell nice.

It's looks like a 12 year old house From 2004 Staging won't hide that.And here's the issue you can't update it easily, It's going to run you probably $30K to repaint entire house and trim, spray all cabinets, retile all bathrooms and put quartz in kitchen and Baths and put in new appliances.

Just buy it and encapsulate with spray foam. 

That's not too bad - they' could've kicked in all the drywall, ripped out the light fixtures, pulled out the sinks/toilets, spray-painted all the windows, poured concrete down the drains & super-glued the doors shut.

The reason for the threat was that I had accused his girlfriend of stuffing a piece of spray foam back in the hole I had cut it out of weeks earlier, after they had complained it was blocking the gas shut-off for the stove. why they were concerned about the shut-off cannot be explained other than looking for something to harass me with.

Sounds like you could rent a rig and spray it yourself.  

No excuses.Provide quarterly pest spray and meet the exterminator so that you can walk through the apartment and change the filters....because no tenant is going to do it for you....even if you furnish 12 months of filters.Get the rent ASAP.