The first problem is actually easy to fix -- make it a rule that anyone that sends unrelated gifs/makes funny banter gets to buy lunch for the others ;-) (or ma

The first problem is actually easy to fix -- make it a rule that anyone that sends unrelated gifs/makes funny banter gets to buy lunch for the others ;-) (or makes a charitable donation in proportion to the number of people -- if the users are geographically dispersed). And enforce it strictly. You'll find that your slack channel is automatically devoid of unnecessary chatter...something about having to buy lunch for colleagues brings out the miser in them :-).