It’s totally OK to block anyone who spams you in any kind of way, be it requests to be friends, asking for ‘help’ (unless for help they want to place an order for for something your gigs offer, obviously), share your buyers/orders with them (yes, it happens), or want to scam you (that’s usually pretty obvious right from the start but Cy may still be right with the social engineering) or communicating inappropriately (harasses you, threatens you, gets explicit, etc.).
If someone is new and just politely messages to ask for ‘help’ (as in "can you give me some tips, can you tell me how xy works here,…) and you feel generous, you could also just tell them that it’s not allowed to message people without intending to actually place an order and that they may get even banned for it if someone reported them and redirect them to the forum (I have saved a “quick response” for that) but even for that, you’re well in your rights reporting them, it’s their responsibility as new users to inform themselves about what is acceptable and what not when they start using a platform, and not yours to teach them.
If the messages about being friends and the profiles they come from look very suspiciously alike, I’d probably take screenshots and directly report them to CS via the ticket system, so they can look into it. If it’s just separate users, they might get away with a warning but if it’s a person with multiple accounts they will (and should) get banned.
Like Cy, I have had a few conversations with other sellers but only if I knew them from the forum before, and only before this new forum with the pm option was installed.