Here are some suggestions to help block Instagram spam posts:

Go to your Instagram settings and turn on "Hide Posts from People You Don't Follow.". This will hide spam posts from accounts you're not connected to.

Report spam posts as "spam" when you see them. Instagram reviews reported content and may block the account.

Block accounts that are posting spam to you. Go to their profile and select "Block User.".

Tweak your account privacy settings. Make your profile private so only approved followers can see your posts and comments.

Clean up your follower list. Unfollow accounts you don't know in real life to avoid spam engagement.

Use Instagram's comment filtering tools to automatically hide comments with certain words or hashtags.

Manually delete spam comments or posts as you see them to remove them from your feed.

For alternative social media platforms to post artwork, you could consider:

Twitter has a large art community and easy-to-share images. Less spam than FB/Instagram.

Tumblr: Visual-based, art-focused. Less overt commercialization than other sites.

DeviantArt: Dedicated artwork hosting. Large, supportive artist community.

ArtStation is a professional platform for digital and 3D artists. High-quality portfolios.

Behance: Showcase projects and works. Geared towards creatives and designers.

I'd try Twitter, Tumblr, or DeviantArt. Clean your privacy settings, use reporting tools, and be selective about who you engage with on any platform.